I think that's a pretty reasonable system you've got there... although I know at least 1 player who'd probably start sabotaging peoples transport if there might be +10% xp in it for him.
Like you, I prefer to play if we're only missing a single player. Seems a shame to spoil an evenings RPGing for everyone else.
At worst case, like a critical player missing, then we'll play a board game, muck around with another system or watch movies - but will still find something to do. Couple of people use RPGs to escape their families for an afternoon - I wouldn't want to get in the way of that
/This is what I do:
I'll generally find a reason for the PC not to be involved in the evenings session - if in a town, they have business. If in a 'dungeon', then they fade into the background.
They get full xp.
What they will lose out on generally is a cut of any loot gained, or having any of their side plots develop.
Figure they miss the entertainment of roleplaying, which is bad enough. No need for their character to fall behind mechanically?
As for the ECL adjustments - I'd go for 'making it up' to the other ECL+0 players in some way. Magical item they can improve cheaply, in game contacts, a wibbly innate power or something along those lines.