Welcome to the forums.
From the sounds of it, I am going to have to agree that it sounds like you are backed into a really hard corner.
AIP 400 is getting really high for a game without Fallen Spire enabled.
I'm not going to flat out say you've lost without seeing your actual game, but you are in a really tough position.
Conclusion :
- is 400 supposed to be the "level of AIP that must not be reached" at 7/7 diff ?
Conditionally, yes. There are circumstances where AIP that high is not an automatic loss, but in general 400 AIP is getting really high.
- are you really supposed to have only a border of one or two systems only at that level ?
Realistically? Yes. I noted you are playing a lattice map and that is making things significantly worse. The outlier maps, so lattice with so many connections, and maps with so few connections (such as snake or vine) are not officially balanced and are provided to give us players more variety.
I am a fan of lattice maps myself, but they make the defensive part of the game magnitudes harder then even realistic-type maps.
- are waves, CPA and so on supposed to be next to not threat compared to, well... threat ?
Threat, and the ThreatFleet mechanic are there to keep you on your toes. Threat is designed to give the AI means to harass you in response to your actions without having to send a whole wave at you. (See ships being freed from guardposts when you attack, etc.)
The issue is that AIP has gotten so high that ships disregard their orders to become threat if you so much as breathe in their direction so ships are just piling onto threat. While this is not really their intended function at this high AIP, it works well enough that I don't think changes are currently planned to it.
Note however that the lattice map type is once again making things harder for you. Even a realistic map would have probably half the number of systems exposed to the AI that your current game does.
- isn't the regular game (not the hardcore 9/9 game) supposed to be play at around 400 AIP first AI / 600 AIP second AI ?
This is subjective to the player and game settings. I'm probably one of the worst players to ask about AIP levels as I pretty much the define ultra-low AIP playstyle. (I once started attacking the AI HWs at an AIP of 20. I ultimately lost due to my fleet being too weak but their is a reason I enable CSGs in my games these days.)
- is there a mechanic I'm missing ? (one that will stop / reduce those reinforcements...) Note that I use the jammer station but with 5 stations only on a lattice map, it's nearly useless. Also, the jammer stations do get attacked by threat, rendering the point moot : unless you litterally cut the AI territory in half, reinforcements will still come to those planets, so blocking their spawn is kinda useless.
I can't think of anything you've missed. Really I think the biggest thing is the lattice map type is what is ultimately causing your problems.
Even the realistic map type, which is the map with the most connections after the lattice map type, would make your defensive situation so much easier as you'd have at worst only half as many border systems exposed to the AI, if not fewer.
Depending on the other game settings, I personally rate the lattice map as adding an extra half, if not a full, level of difficulty to what you actually select in the lobby.
A couple other notes from your opening post.
Core Shield Generators: You need to take 8 worlds to destroy these, not a dozen like you mention. I suspect you know this already and were not being exact when posting, but I just want to make sure.
The wiki saying 400 to 600 AIP is okay: I just found that page and ouch, that needs updating. That comment is from early 2010, there have been 2, if not 3 expansion packs released since then and the game has changed significantly.
Other then that, I'm not sure I have any other advice to offer. 400 AIP is high for sure, but you mentioned golems so it might be possible to punch through it, but the lattice map is going to make it easy for the AI to flank you.
Good luck.
D.
edit: You posted while I was posting. I will say that I think over time the 'acceptable' AIP level has certainly decreased as changes have been made to the game.
Just in the past couple weeks this has been commented on (see the AIP Restrictive at high levels thread) and things like the Lazy-AI option which just got added last patch are at least partially intended to address this.