Consider that this game is the second installment and for this price, for the size of the studio, what can you expect?

Yes, 100%. I played DD1, don't remember at all what it was about and still enjoyed DD2. There were some mentions to DD1 events but the story is pretty self contained.

Considering that you play on the default difficult(Hero) you should take about 20-30 hours.

30 main chapters, 6 Challenges and as many Training missions as you want. I played only 1 training to check what it was about, which is simple leveling chars if you want or need.

In default difficulty it is on the easier side, so no min/max required to optimize teams. Just pick who you like and the class you like and have fun.

In default difficulty no, in average you will finish close to half of the turn limit.

Yes, a lot in fact and they change some of the content but not much. So not much of right or wrong.

Yes

Yes and yes. Consider that the game ends the story and hints to more but you do not feel as some other games where the story ends to quickly or the main quest is not addressed.

You will find a lot of good chars to play in this game and everyone is good on their on way, so let me clear some questions here.

Could not find one, everyone was good. From growth tables available everyone looks pretty even.

The army will hold 20 characters by the end of the game but you can only deploy 10.

None!
Nobody really leaves or dies in any relevant way during the story.
At the beginning you get some some chars with a few extra level but they are super good to use, unlike the promoted units in FF.

During the last chapters my chars were between 40-45, not sure if there is a level cap.

The characters start in their base class, than move to Tier 2 at level 5+ and Tier 3 at level 25+.
Yes, do promote as soon as possible, this increases the Stats gain and grant access to new passive and abilities
Yes you can change your char class after promotions. But be aware that after promoting to Tier 3 you can't change the Tier 2 class anymore, and this is hugeeeeeeeee.

Yes! you can build your char to have 100% or close to in Crit and it is a very common thing to consider for most if not all of the chars.
Consider that Crit double the damage and some skills and runes can increase the Crit damage further.

Yes, a few chars can be built to be evasion tanks and have 0% or close to in hit rate.

At Hero difficult not really. poison, bleed and others can help a lot but not that much.

There is no STR or MAG, damage is based on Might + Weapon = Power or pwr in the char screen.

No, they provide gold, gems and story content. The only exception is if you have rings that rely on Bond related things.

Considering that everyone is good and viable to use, what makes them stand out?

• Base Class - Some will become more Magic related some more Physical related damage
  
• Base passive and ability - Everyone comes with a default passive and ability that never changes, for instance Gwyn comes with "Ferocious Advance" and "Brutal Strike"
  
• Bonds - Everyone will have up to 4 bonds and you might want to consider this for the money+gems and story

None

Some battles if you attack with a particular unit there will be some dialog but nothing else.

There is no perma death, if a char dies you get a random penalty for the next chapter, which in general are pretty light.
Except for Gwyn, if she dies it is game over.

DD2 handles weapons a little different than other TRPGs so it is worth making some notes on this.

Each class can only wield one weapon type, changing the class can change the weapon type.
There are Swords, Dagger, Axes, Bows, Focus, Relics.
There is no concept of Swords beats Axes such as in FF.
There is no concept of weapon proficiency, weapon level or anything that makes you sticky to the same weapon or weapon type.

Never really, during the game you get new weapons that will have different damage/crit etc, but in general all the weapons are good

They are not, you will notice that the status changes but most important the runes that can be used for each weapon are different. So you will have weapons that can drain HP while others can't.

Kinda but not really, you will find enough during the game and you can also buy at the shop, you can also recover used runes for a small fee. So use it all the time!

Rings are equipment for you chars that allow them to have additional effects or change the play style completely.
Notice, you can only craft one ring of each kind

Some what hard, you will get some during the campaign and you can buy in the store but it is not like you should craft all the possible rings easily.

No clue. They dependent on the char and your play style. So if you have a char with a lot of bonds you will find the "Love Ring" useful, "Hurricane Ring" is always good with +1 movement, but I'm sure you are smarter than me and can see different combos using rings.

No, those stones are only for Rings.

In my playthrough I found Diamonds and Rubys to be rare, but I know I can always buy more in the Store.

Thanks for reading and if I made any mistake or you would like to add some info just post in the comments and I'll update this humble starters guide.