We haven't used any XML checkers - well, at least I haven't and I haven't heard of anyone else doing so.
The .xql files are mostly automatically generated by the query extractor (see the link from the porting HOWTO, reachable from http://openacs.org/4). So they're error free.
The query extractor also does a fair amount of the
translation of Oracle-specific queries to the Postgres or SQL92 equivalent, and flags many other that it detects but can't translate with a FIX ME tag.
So in practice mostly we haven't had to worry about the correctness of the XML in the .xql files, just the actual queries themselves.
You've been looking at the CMS, though, and unfortunately the aD team that wrote the CMS used their own set of utility procs to do a bunch of stuff rather than those used by the rest of the toolkit. This was due to the fact that it was developed in parallel with the ACS 4.x core, mostly.
The query extractor doesn't handle this custom API so the folks porting the CMS have had to do a lot of hacking on the queryfiles by hand. So they've had to deal with a lot more potential for error.
As far as editors go, most folks here seem to use emacs. I still use vi, but then again I'd probably still be using Unix V7 on our old PDP-11/45 if it hadn't been junked ten years ago :)