Robyn,

I will not use soil in my bog; just peat and sand, 50-50...

Can you tell me if you think that the following marginals will do well in soggy peat at a low pH:

-Arrum
-Pickerel Weed Plant
-Arrowhead
-Sweetflag

Not having used peat myself or researched it, I don't know the answer to your question for sure. I know of some plants that like the low pH, so-called bog plants but those aren't in your list. Sorry.

they say that i should avoid adding dirt - with peat it, supposedly, wont smell much and will be much healthier

that many bog plants do not like or require a lot of nutrients, i.e. CPs/fly-traps, etc.

i think that if the liner were to stop 6" before ground level, there c ould be no real problems with smell, etc - water would not only never stand, but not even the upper bog earth would be more then moist - and the water that is trapped will be inches underground.