this civil war is honestly a slog but is winnable and requires a somewhat deep understanding of the game, here are some things to keep in mind
1. never. ever. ever. get cut off from your capital by key lake, your troops will never get resupplied
2. understand where key resources are, like aluminum and steel you will only have so few civs to trade
3. take your time but not too much time, you will be on the back foot for the majority of the civil war with insufficient number of troops to fully defend your border and struggling to hold what you got, bee-line it to the focus's that grant you units and weapons
4. beware of olenia they will most likely declare on both you and chrysalis if the civil war drags on more than it should, which is both a short term blessing and a long term curse, considering you will be able take most of the changeling lands as chrysalis struggles to fight a two pronged war and by the time you are fighting olenia you are hopefully managed to take a ton of core land and mount a successful counter attack, but here's the catch you shouldnt annex them considering you have a focus to start a harmony coup and eventually puppet them and annex them peacefully and core all their land which oh my god do i love all that manpower.

Micromanage a ton, encircle I normally play on five speed for every war (because my computer can cope with it) but had to lower it to three to micromanage for this war. Don't go for the centre go for the north and south because they are generally more lightly defended.

Oh and go for the focuses which give you addittional divisions and then equipment before all else. Hope that helps

Yeah,

Harmony Changeling is so far my favorite playthrough although I just did it 2 days ago so it may have changed from this original thread. I set up for Dawnbreaker and King Sombra to trigger her. I made it through the civil war pretty quickly. Too quickly to get the extra factories and resources, but I felt like it could speed along unifying with Olenia which provided me with more and more quickly so I went for it. Plus it's fun to 'play it as it lies' sometimes. If you're min maxxing drag it out for the extra factories and steel by weakening the enemy but refusing to go for the killing blow.

The advice given above is good. You give ground in some places so you can form overwhelming encirclements in others. I wasn't perfect, but I didn't lose any divisions. I utilized a tank division in the south and Jagers supported by infantry to get a really good encirclement of a bunch of motorized and a tank divisions that struck in a little too deep and aggressively. I gradually lost my original capital in the north, but it was tough for the enemy going through those mountains and the new capital was well positioned and the enemy got really bogged down up there. So I won in the south and sustained the middle. It caused me to eventually have an overwhelming fast moving force coming up from the south while the enemy got stuck in my mountains and so I just captured more and more divisions from the south on up.

The focus that gives you units is a huge shot in the arm right when you need it. Once I got it I basically took Vraks then Vesalipolis. Unified with the two stranded units in the west and rapidly unwound everything. It was over by July. Notably Vraks and Vesapolis are weakly protected so sometimes you can even rush them. I didn't though. I just focused on gradually ending up with more divisions left in the field.

Make use of tiny division templates and insta spawning untrained units. Also feel free to strategic move your units and lose all organization to get them where you need them. You can clean up your templates later after you've won.