Missing null.pcx is caused by not having the OpenTDM "invisible player" model installed, it can be found on SVN or in the server download (I think!).
Choppiness has nothing to do with the mod since it's all server-side, there is no way for it to cause any kind of FPS problems or similar.
Stats are already preserved for the last game, they can be accessed by oldstats / laststats / oldaccuracy / etc. All stats commands also take an optional player name or ID too to see other persons stats.
Looking up / down bug on spawn is very rare bug from 3.20, it happens in almost all mods at some point or another. Probably caused by delta angles messing up somewhere, need to research this further.
The way items work, Megahealth is just considered Health with 100 count and special think handler, changing the name would also affect regular Health packs unless a gross, ugly, disgusting hack was added