I’ve never done a purge for the loot that drops, only for the unique named thralls and the challenge of completing it. The new system seems broken in some regards, as mentioned in the (main post) bullets above. The decretive chest in the FOB cannot currently be interacted with. Once Funcom fixes that, we will have a better idea of what the rewards will be. I don’t think the rewards should scale infinitely though, perhaps just the first 10 (or 20) levels could scale, beyond that seems excessive to me.
I wish there was a console beta for people to test, but unfortunately, I don’t think there ever will be. As far as lag, I agree with you, server lag is likely going to be a bigger issue than it ever was, since everyone can run a purge simultaneously. However, I think my suggestion keeps the lag to a minimum with only 10-14 NPC (AI) per wave and the total waves never exceeding 10.
There was (Dev) talk about siege weapons being used in previous streams, and I know most of us jumped to the conclusion that they meant trebuchets, since those were the only siege weapons in the game at the time. However, some of the new purge thralls carry battering rams and those may have been the siege weapons they meant.
I personally hope more siege battles can be fought in chapter-3, with trebuchets and possibly ballista or scorpions. We will have to wait and see with our fingers crossed. 
My thought, which in hind sight wasn’t articulated well, is that you are forced to proceed through the threat levels one at a time (sequentially), preventing someone from skipping straight to level-30. However, once a level is complete it is unlocked and that clan can then choose to do whatever level they wish out of the unlocked levels available to them. Perhaps a drop-down selection box can be added. Being able to select the level (from unlocked levels available) is important, but most of all for large clans. A large clan may have achieved a level-75 while all 10 clan members were online, but if a single clan member wants to do a purge while his buddies aren’t available, he needs to be able to select a lower level.
I agree, the loot should scale for the first 10 levels, and I suspect it will once they fix that bug with the decretive chest. Named thralls should also be part of the loot though, as you mentioned, I haven’t seen any named thralls yet.
Perhaps every 10 levels you get a guaranteed (random) named thrall in one of the cages, up to a maximum of 3 guaranteed.