Would it be possible, a huge amount of work etc. to set up a band-width/cpu constrained, version of PM running on (one of) the existing server(s) that could have patches applied by lesser mortals than the gods, so that they might be exercised in a non-critical environment?

We have one of those already, but it too is gods only, because both the code and all data lives in the same database. I don't know how much of a problem it would be to separate the code so that exact functional duplicates of PM could be set up, but it too needs someone to do this work at the very least. (I might look into it later tonight or maybe tomorrow.)

Another discouraging problem (for newcomers anyway) is that it's not readily obvious if a node is in use or contains dead code, and we have a lot of the latter..

I'd also bet that besides a few gods, noone really understands more than a small portion of the interactions implications of the complete engine. tye in particular has spent a lot of time optimizing PM's engine flavour and pulling at its innards. I doubt there are a lot of people who can do remotely as much as him or a handful of other gods, which is why I tried to make a move for a tye fund.

Besides that, the most practical and most promising but also least feasible thing I can think of that we could now to alleviate as many of the problems as possible would be to try and migrate PM to a current version of Everything, so EveryDevel and PM can benefit from increased synergy. However, it's nearly as large a project as a full rewrite of a new engine..

Personally, (as I've said a bunch of times in the last couple of days) I'd really prefer if no code were stored in the database since that would allow working with standard tools (of which CVS or cousins would be very desirable), but it's not likely that PM is ever going to (be) switch(ed) to such. So being able to share work with EveryDevel seems like the most practical way to generate the synergies we'd need to bump along faster.