AFAIK the DIN-5 <-> Mini-Din-6 (PC to PS/2 keyboard) adapters are just physical connector adapters, and the keyboards (at least from the AT/286 ones onward) are electrically and logically compatible. The PS/2 <-> USB keyboard adapters are a USB HID device on one side, and a PS/2 keyboard on the other side, and I believe convert the PS/2 serial events (keyboards are slow, sync, serial) into USB HID events on the USB bus (and presumably vice versa).
So to the best of my knowledge the PC keyboard -> adapter -> PS/2 -> adapter -> USB chain should work just fine. One day I'll have to find out; I've been using a AT keyboard since I owned a 286, and it's getting moderately common to find "legacy free" PCs these days with no PC or PS/2 keyboard ports.
For the "hybrid" approach you mention (some apps remote, some local) you might want to look at LTSP (http://www.ltsp.org/
) -- I believe they've got some wrappers for the "run displayed remotely" and "run remotely displayed on that console" situations. (They also found that video playback, etc, i best done locally on the relevant machine.)
Finally a dedicated network link between the compute server and the (X) display server definitely helps with latency issues on running X applications remotely from the display. Especially -- these days -- if it were a gigabit connection. But it's still be noticable with all the badly written, latency sensitive applications. (That said, Firefox is much better run remotely than, eg, Netscape 4, ever was.)