Take-offs and landings only come ito it for the purpose of the 90 day rule and since there is no requirement to log them it's somewhat difficult to prove whether or not you complied. I normally put "x landings" in the remarks column if there's more than one and if I wanted to be pedantic I'd put "no landing" in remarks if someone else did the landing.
I think some new JAA logbooks now have a separate column for take-offs and landings. In mine I made a small separate column for them after the Remarks column. I need to know, as I fly fixed-wing aircraft quite rarely, and struggle to stay within the 90-day rule, so I find it very useful.
As for splitting one flight with another pilot, I've done it several times on long trips on the continent, if for instance we find that we're flying a particularly long leg. Each of us logs the time we actually flew, and makes a note in the Remarks column explaining the whole thing, eg "X flew from Y to Z and landed", or something similar.
Common sense is allowed in logbooks, honest!
As for what seat you fly from,
AFAIK in some aircraft (such as the R22) you have to fly it solo from a certain seat for weight and balance reasons. Also, some flying schools or clubs or groups may make their own rules. Otherwise you can sit in any seat and fly it as you like.
