Nyeshet said:
So it is impossible to determine the CR of an encounter involving, say, 8 Half-elf Fighters (half 2nd level, half 3rd level), 1 Human Wizard (8th level), and a Wight Fighter 4 (CR 7)? Granted, you would use an Encounter Level calculator, but if the Encounter level has a given CR that could result in an xp award then xp should be granted, I think.
Encounters don't have a CR, they have an EL. (The confusion comes because an encounter with a single monster always has EL = CR, but that's coincidence.)
4 2nd level half-elf Fighters = EL 6 (2 for the first fighter, +2 to double, +2 to double again)
4 3rd level half-elf Fighters = EL 7 (3 for the first one, +2 to double, +2 to double again)
The Wizard = EL 8
The Wight Fighter 4 = EL 7
Two ELs 7's = EL 9 (7 + 2 for doubling)
According to the DMG, an EL 8 + an EL 9 = EL 10
And then adding an EL 6 isn't quite enough to make it EL 11.
That's a very rough calculation (since the rules don't really cover combining four CR 3's with a CR 7) as I have done, but since CR is a rough tool anyway, it's probably good enough.
A party that defeats such an encounter will gain 4 times the XP for CR 2, 4 times the XP for CR 3, the XP for CR 8, and the XP for CR 7. This may or may not equal the XP for CR 10 or 11 (probably not).
The thing is, as has been mentioned earlier in the thread, large numbers of very low CR creatures don't make a meaningful challenge for a high level group. One well-placed
fireball can easily take out a dozen Orcs, while a properly-equipped Fighter likely has an AC beyond the capabilities of the Orcs to hit more than a few times. In effect, then, any creatures with CR less than 8 below the party level can almost be considered not to be a challenge at all, regardless of numbers, and so aren't worth any XP. (Unless, of course, you use the Mob template or equivalent.)
Whether Belkar would gain role-play XP is for the DM to decide. I don't give out such things, primarily because role-play is essentially its own reward. But there certainly is a strong argument for doing so.