After scouring the web, I couldn't find any info on certain game mechanics, so had to figure them out by trial-and-error. Hopefully this will save other players the same trouble. This is hardly comprehensive or complete, but should help at least a little.

Health
In Gensokyo Warfare, your max HP is NOT the number shown on the character screen. Instead, this number gets multiplied by a variable (which can be found on the Camp screen). This scales up with level, along with a few other hidden factors.

Cosplay
Cosplay has a very important mechanic underlying it that isn't mentioned anywhere in the game. Specifically, get a very large bonus to all your core stats based on how much the owner of the outfit likes you. At relations 100 this is: +222% HP, +74% damage, +54% accuracy, and +18% speed. Note that this bonus is from wearing the full set (usually two items, though some characters have more). Cosplay outfits are obtained by getting your relations high enough with a character, after which a dialogue option will appear to ask for their outfit. It's worth noting that the max bonus is the same across all characters' outfits, regardless of how powerful they are in-game or in lore.

Concerning Touhou Characters
Faction leaders and a few VIP characters are extremely powerful. We're talking like over a thousand hitpoints, plus running faster than a horse, and will kill anything (even you at high levels) in a few direct hits. Although it may seem cool to have a duel of superpowered leaders, don't even think about it. It's far safer and easier to kill them through sheer numbers: get them swarmed so they can't effectively string their weapon, and are immobilized from flinches and knockdowns. Then you can maybe speed along their demise by hitting them in the back over and over again while they're focused on the other troops.

Most deadly of all is Remilia, since she has access to the Spear of Gungir, which is basically a massive thrown bomb doing 250 damage in a massive radius. We're talking like knocking out easily 50 units at once (even the strongest ones in the mod) if they're clustered together. If you must fight Remilia, get into melee combat ASAP so she will be forced to switch from her throwing weapon to a melee weapon, stopping her from bombing your army into the stone age. Don't let her pull back and switch back to throwing weapons!

It's also worth noting that in the current 0.5.0+ version, getting any character to defect from their native faction is virtually impossible, so don't waste your efforts trying. This is because this version introduced offices, one of which will increase vassal opinion of their monarch so it basically rests at near 100, no matter how unfair or incompetent of a monarch they may be. So they will never be of low enough opinion of their lord to defect. The only exception to this rule is if a faction is destroyed, and the vassals scatter to join new factions. In that case, the newly-arrived character starts near 0 standing to their new monarch, so will be vulnerable to defection. So if you see a faction rapidly shrinking, do your best to befriend them as much as you can so you'll be in a perfect position to get them to defect if they don't already decide to join your kingdom when their faction falls.

Concerning Kingdom Building
Due to the above issue of defections being near non-existent, starting your own kingdom is drastically harder in this mod compared to most others. The lack of nobles willing to join you will be crippling disadvantage, leaving you with two choices: Reduce your own party's fighting power by converting companions into nobles, or waiting until an existing faction is wiped off the map so there's a number of disloyal/persuadable leaders available.

The best place to found your kingdom is the Magical Library, for four reasons: 1) It's a massive grain producing location, which means you'll have huge amounts of very cheap grain and bread available for feeding your army, in turn reducing army upkeep costs. 2) It has the most productive hinterland (attached villages) on the map of any city, making it particularly prosperous. 3) It's almost the exact center of the map, reducing travel times to distant locations. 4) Except for the four human town quarters, it has the closest concentration of cities on the map. This gives you great overlapping patrols and quick response to sieges.

Race
Besides your starting location, this also has a hidden effect: You get a bonus to max HP based on your race that scales with level.


From my testing, there virtually no difference in other core stats like damage or speed.

History
This is effectively starting difficulty. It does not, as some options seem to imply, have a big impact on core game stats like the above race choice options. The rest of the choices in character creation are largely similar to vanilla, giving slight increases to certain attributes as far as I can tell.

Which faction should I choose?
If you're a new player, you might be somewhat overwhelmed by the extensive troop trees of various factions. Here's a simple guide:

• Scarlet Devil Mansion: Focus on armored heavy cavalry, but consequently, they're pretty weak at long ranges due to their higher-tier archers (mages) using multi-shot burst attacks instead of precision single-shot ones.
  
• The Netherworld: Heavy emphasis on lightly armored but very fast units. They have a special mechanic where you can purchase Himepaper (an item) from a tavern seller, and use it to conjure troops on demand, through a camp menu option. They also automatically generate "ghost" units (not much better than peasants in terms of strength) every time a unit dies, be it friend or foe. Units are very specialized for certain roles, and less flexible than those in other factions.
  
• The Underworld: A faction that appears to favor light, fast cavalry with ranged weapons to harass the enemy while on the move. They only faction that has access to horseback grenadiers, which also have the strongest grenades available. Alternatively, also have Oni troops, whose slow but extremely high damage per hit attacks makes them particularly deadly against (normally heavily armored) elite units or hero characters, though less effective against large numbers of more conventional enemies.
  
• Myouren Temple: A large proportion of units use blunt weapons, which naturally makes them good at capturing enemy troops alive. Otherwise an uninspired jack-of-all-trades faction, as far as I can tell.
  
• Youkai Mountain: Most notable unit is the Kappa tree, which provide a deadly combination of heavy armor, rapid fire crossbows, grenade launchers. Their musketeers also have double the normal ammo capacity, even if they aren't as precise or fast. Aside from Kappas, there's a heavy emphasis on pikes, making their forces particularly deadly against cavalry.
  
• Hakurei Shrine: They specialize in ranged combat, with a secondary focus on polearms as their only melee weapons. The combination of polearms and ranged fighting means they perform well in wide open terrain, but poorly when forced into close quarters and sieges. For range they have have archers, multi-shot archers, needle throwers (which are so fast and accurate, they are effectively muskets), amulet throwers, heavy orb throwers, grenade throwers, and so on. Note that even through they all wear miko robes, their heavier melee troops have the same armor level as the plate armor wearing units of other factions.
  
• Senkai: Zombies! If you have a Taoist Onmyoji or Master in your army they can resurrect those killed in battle as zombies, once the battle is over. Zombies in general are extremely durable compared to most troops.
  
• Eintei: Guns. So many guns. Literally every unit in the core tree has a gun. Eintei has access to snipers, the deadliest range units in the game. Also have shotgun cavalry. Even the melee troops have semi-automatic pistols. No one will match Eintei in ranged DPS, but their complete lack of armor on most shooter units means they will melt away pretty fast when engaged in melee combat, or even at medium-range where other (more heavily armored) factions' archers can reach them.