This mostly matches my experience. By far the most intense version of this I've had was the time I tried to play Chess and Go simultaneously (against two different people). I started sweating and shaking. This seems to suggest that not only is thinking a physical effort, you can push yourself much harder under some conditions than others (just like how deadlifts will physically exhaust you much faster than pushups, even if you push yourself to do pushups as hard as you think you can).
One of the functions/problems of funeral rituals is coordinating the direction support needs to go - people support people who were closer to the deceased/are having a harder time, and get support from people who are having less of a hard time.
I guess this means a funeral is a two-group event, at least along that axis - you have the group of people being comforted (family and close friends), and then the group of less-close acquaintances, who (aside from being there to deal with their own grief) are also there to comfort the first group (both by direct action, and by showing them the person they lost mattered to people).
I guess the implications of that are (a) sometimes (like with your friend) you need separate rituals, because you have multiple important first group/second group divisions. And (b), it's not only okay to be there if you didn't know the person that well, it's important (since you need the second group). And from the outside view, you should expect most funerals you go to to have you in the second group.
In terms of the ritual, I'm not sure what the implications are. Maybe it suggests that if you don't have a direct fit for the deceased's wishes, you should look for something representative of group 1 instead of the general attendance (though this raises the problem that identifying group 1 isn't easy - the roommate the person moved in with two months ago may be either a total stranger, or closer than their estranged family). It does suggest that the ritual needs to leave room for unidirectional comforting, but that seems easiest to do by leaving unstructured communication space.
I'm interested. I'm moving to the bay (work in MTV) in August.
(I'm also interested in group houses and like kids, so if there's a shortage of grouphouse pro-kids people I totally have comparative advantage there).