After testing and reading the raws and wiki I can confirm the following:
1. The siege will 100% happen on the 3rd. We didn't get any until my reign because we never had any children in the fort that to be snatched.
2. The siege make up will vary. I had one run without any gobbos on foot which was !!Fun!!
3. Opuntia, rainbow, etur and myself are all strong enough with exceptional and masterwork steel now to break entire squads of goblins by ourselves but we still die easily to arrows. More importantly, it seems the archers are the main enemies, we seem to be able to crush anything in melee through Zygarde or a good old martial trance
4. Zygarde can and will be killed if its caught in the open though the siege will leave afterwards but it takes at least 2 squads of archer gobbos and a good melee squad of goblins ot hold it still enough to bleed out. Melee goblins struggle to do much damage to Zygarde since its got scales made of slade
5. very few trolls seem to be coming. Over 7 sieges of testing, I only saw an average of 8 per siege.
6. Our crossbowdwarves have a habit of emptying their quivers then going and grabbing more bolts from the battlefield rather than the ammo stockpiles. Twits.
7. We apparently get goblin poets invading
8. By shutting the drawbridge, the trolls end up caught in cage traps first thereby stopping the goblins where they stand.
This is beneficial as I have noticed that when the wildlife comes close, the voltorbs...tend to hurt the gobbos as much as themselves. By leaving them outside they keep hurting their own forces
9. If the gobbos can't get in, they do get bored and leave after a while without fighting
Finally, I did some testing and even with invader caps and even the invader flag turned off, the siege still starts. not sure if I'm missing something or its just too late to disable this siege.
We do have an illegal way of smashing it to pieces though- the exterminate function in DFhack to execute all gobbos, beak dogs, trolls etc
For now I'm going to keep testing and see why this flag isnt working as it should.Edit: Ok, I have good news and bad news. The good news is for future sieges we can change the files. Tested this afternoon with invaders and caps on and no invaders. No invaders worked wonderfully even after I incited war with the elves. 2 years of gameplay and no invasion of goblin, elf etc.
Capped invaders were a bit more finicky, not all species were restricted. gobbos and beak dogs were
Bad news: we can't do shit about this siege, its already loaded in. We're gonna have to take it on somehow
Whichever solution we go for, if for the future we are restricting numbers, then we need to do it now. I can make a new save with the D_init altered