From how inactive the forum is, temporary karma wouldn't do very much, but what we're talking about here sounds like a karma system and a thanks/kudos system at the same time, which are different depending on how they're implemented. Karma as I see it is mostly a forum wide thing and is completely unrelated to posts, whereas kudos are post individually but the total may be shown on the user's profile. Kudos probably shouldn't expire, but karma could. This is really probably just a matter of semantics or splitting hairs, but I think the described kudos system would work better than a positive/negative karma system. A community as small as this one would do well with as little negativity as possible.
As for the questions, they're mostly answered, but I'll give some clear answers if you do the kudos thing.
One kudos from a user per post
Shouldn't return to 0 in this case
Bottom of post would show how many/what users gave kudos
Yes
Additional note: total kudos could be present in user's profile.