1) Is there a easy mechnisim to give back AS numbers to the ARIN?
Extremely easy, yes. One quick template.
2) Would ARIN even reallocate those AS numbers to new requesters?
Depends on the circumstances. I believe ARIN would need the returning organization's permission to re-allocate the returned ASes prior to one year or so.
3) Could those AS numbers be transfered over to new organizations?
ARIN doesn't allow others to act as registrars. Unless there is a merger or acquisition type activity, neither address space nor ASes may be transferred.
4) Is there a advantage for a organzation to have multiple AS numbers?
I'll let the big boys speak to that.
5) Is there a advantage to having a lower AS number?
Some network operators measure the size of their, um, "networks", by the lowness of their primary AS (Scott Marcus-envy). :> /david (just some schmo who barely even represents himself, much less others)