Putting the Band Back Together seems to be solely reliant on the RNG making Germany oppose Hitler and on Hungary putting Otto on the throne, which are two things I haven't seen the AI do simultaneously. I've seen the AI restore Austria-Hungary loads of times when I'm playing as Germany (most often after I kick Hitler the the curb) but I've never seen the AI hit both in the same game. Come to think of it, I almost never see Hungary restore the monarchy unless I'm playing as Germany.
Normally, adjusting the game settings deactivates achievements, but I wonder if the devs are going to set it so that the player can manually change Germany or Hungary's behavior at the start, to prevent us having to start a game, see if Hungary puts Otto in power, restart, rinse, repeat, etc. I mean, this doesn't seem like a difficult achievement to get, mechanically. It just seems 95% reliant on the RNG deciding that it loves you today.
A lot of people are misreading this achievement. Although thematically it makes sense that it would want the Kaiser instead of the Nazis, the description just says "Germany". So you can join the Axis (or conquer and puppet Germany, or any other more exotic way of being in their faction) and fulfil that requirement without any further reliance on the German AI.
For Austria-Hungary, it will depend on how exactly it's scripted but I would assume it would accept equally:
- Hungary on the Habsburg Prince focus tree path
- Austria, Czechoslovakia, or Hungary under the influence of the German "Rekindle Imperial Sentiment" focus
- Austria, Czechoslovakia, or Croatia having taken the baseline decision to form Austria-Hungary.
The last one suggests to me a strategy which, while not exactly easy, would require only minimal reliance on the AI.
1. Must be on non-historical (because the AI won't form nations on historical mode)
2. At some point, conquer Yugoslavia. In the peace deal, liberate (NOT puppet) Croatia and feed them at least the northern part of Yugoslavia (pre-1914 Serbia and Montenegro are safe to take for yourself or make into puppets)
2b. Alternatively, form a faction with either Austria or Czechoslovakia beforehand and invite them into the war with Yugoslavia. Feed them the same territory you would have given Croatia.
3. Conquer Germany (easy, right?) and puppet them. Feed Austria and/or Czechoslovakia to your faction partner.
4. Clean up any more Austro-Hungarian cores still left over and feed them to your ally.
5. As soon as the AI has the required territory it will take the decision to form Austria-Hungary and that's achievement unlocked.
Now, obviously this is untested as the patch isn't out yet, and I'm not 100% certain about whether the Croatia manoeuvre will work, but this is a starting point and doesn't seem too egregiously luck based.
ETA: Also, having gone back and checked the Turkey dev diary there is indeed a "Press the Austro-Hungarian Claim" focus in the Ottoman branch which sounds like it could be a fourth way of forming Austria-Hungary ready made for Turkey, rendering my elaborate plan totally moot.