Yes, or differently, elapsed is the time that BOINC is not suspended and a task is switched on to use any spare CPU cycles, which is again less than that On/Elapsed time depending on how efficient a device is and other momentary loads.

The reason that the developers adopted the Elapsed was that the GPU cards/sciences as of yet have no concept of timekeeping, so the wallclock time is used to compute the contribution. GPU jobs either run all out or not at all, so to them Elapsed time is equal GPU time.

edit: spelling.

Correct, no matter what GPU job, some cpu time is needed to interface between the different parts of the system. Some GPU projects use very little, a few percent, some use a bit more. Most of the cases a CPU core handles 2 tasks when e.g. WCG and Collatz is run. On a regular quad with 2 GPU cards you could thus have 6 tasks running. There are also GPU projects where the full CPU core is reserved + the GPU, so only 4 tasks would run on a quad... that is, what I've been reading around. I could be wrong.