1. Oxalic Acid - as you mention holes to be plugged, I'm assuming you're thinking of Vapourised Oxalic Acid ? If so, then - if you fit an OMF (full or partial) - then you'll need some means of blanking it off during the treatment. A plank of wood strapped underneath would be adequate as it would only need to be in position for 15 minutes or so. If no OMF, then simply block-off the entrances. BUT - you'll also need some means of actually getting the VOA into the hive, and that will depend on what kind of applicator you intend to use.
2. Inner Covers - very much a personal choice. For example, Michael Bush swears by 'soft' covers, I happen to prefer hard ones. My reasons are outlined in: http://heretics-guide.site90.com/beek02.htm
which, although a page concerned with a vertical hive, my reasoning (rightly or wrongly) applies equally to my horizontal hives. I would stress though, that many beekeepers use soft inner covers successfully - I have in mind Michael Palmer, who simply cuts a flap into the material, folds it back and places the feeder over the hole thus created. Maybe I'm just being over-cautious ... ?
3. Follower Boards. Much depends on what they're being used for. If you have more than one colony in the box, then clearly you need the follower board (acting here as a partition board) to be bee-tight all round. This is fairly easy to achieve with (say) a Kenyan Top Bar Hive, due to it's wedge shape.
With a parallel-sided hive it's much more difficult. Personally, I'd never keep more than one colony in a parallel-sided hive with a movable divider, precisely because of this difficulty. ( I DO have several Long Hives in multiple occupation - but these have either permanently fixed dividers, or dividers which slide into fixed-position slots to achieve 'bee-tightness')
So - with just the one colony, all that's needed is to create a 'thermal curtain', with the top and sides being 'sealed' - the bottom no longer needs to be. In fact it's desirable to have a reasonably large gap at the bottom, so that any bees which find their way through an imperfect seal can then easily find their way back, rather than be trapped in the unused section.
The type of 'thermal curtain' boards I now use can be seen in: http://heretics-guide.site90.com/beek15.htm
(the last picture), being the invention of Charles Dadant, which he describes in his book 'System of Beekeeping'.
4. You could, I suppose - I'm not a great fan of entrance feeders, as my experience is that they can start-off robbing - so I don't use them. My preference is to use overhead inverted jars for syrup, fondant, and damp-set sugar. Ideally, these do need a feeder-shell of some kind to keep the sun off the jars when feeding syrup. Like so many things in beekeeping, it's a personal preference thing. :)