Excellent tips on getting those injectors out, hopefully they can help someone else avoid this kind of mess all together! Glad to see others share my disappointment with this quality of head work RabbitJockey, $500 should have easily been enough to get me all new valves in a rebuilt head...
I got the head welded a day ago, came out great. My welder said the aluminum in these heads welded smooth, and there was very little oil seeping out. We were also able to keep the threads aligned, and he took special care not to blow through and risk distorting the threads or the sealing surface in any way. Furthermore, he built up the area around the boss with weld so it should be stronger than ever. I've attached a picture of the work below. I measured the head and it appears to be nice and flat still, not even a 0.002" feeler gauge would fit under the straight edge at any point. That's certainly a nice bonus which I wasn't expecting, but I'll drop it off at my machine shop to have them double check.
My prechambers came out pretty easily with a dowel and hammer, and with the carbon cleaned off they slide in and out with light pressure. With this in mind I'm assuming oversize chambers is the way to go, or is standard size with a dab of loctite and peening sufficient? I'm off to school again for a few months, so I have until at least March to order parts and decide what I'm going to have the machine shop do. I will of course update everyone on the situation with the old head and the vendor in question as soon as its over and done.