Philtre

Regarding seeds: harvesting crops gives you a very, very low chance of an extra seed. You mainly have to find more wild plants. One tile of water should hydrate a 7x7 square of farmland, so you can have your farm self-water without having to make it *that* swampy.

Removing farmland will destroy the block. There is no way to turn it back into soil; this is deliberate, to prevent players from gaming the soil-fertility mechanic. There is a mod that allows you to dig up farmland and get the soil block back, if you are OK with mods.

For a structure to count as an "enclosed room", there are a number of requirements, one of which is that the interior space must fit entirely inside a 7x7x7 cube. I'm betting your peaked roof is causing the room to be too tall. I had the same problem when I first replaced my flat roof with nice tiles. Either add a ceiling and make an attic space in the roof, or fill in the portion above the 7-block height.