The shorter lifespan of AGM
in typical cruising boat usage is not due to the amount of electrolyte. It is a function of the typical cruising boat not being able to charge them up fully every single
day using a proper charging
lifespan suffers from partial charging.
Yes, you are way overstating the gassing of flooded batteries. There should be very little gassing at all. If there is significant gassing, something is wrong with the charging system. We live with ours in the saloon
and have never experienced noticeable gassing. Since they only have a potential to gas when being charged, you won't have to worry too much about it while you are sleeping.
In the event of a failure that causes cases to split open, then yes, there will be a puddle of acid. Batteries should be contained in a box for this reason. Splitting open is rare - I have never seen it personally.