It is possible that you have a tire going flat and that is lowering the entire mower on the affected side. It is also possible that the side of the deck that is cutting the LOWER height of grass might be lowering itself due to some adjuster for deck height having vibrated loose and thus is adjusting that side of the deck down due to vibration. Whenever considering deck height adjustment ALWAYS make certain all tires are inflated to OEM specifications BEFORE touching anything about adjustment procedures.

They're bedrooms, so they are somewhat isolated and have a distinct (and small) separate threshold. That's a far cry from the contiguous floors of a family room/living room/entryway/kitchen type situation where a threshold and height difference would be more awkward. I don't think it will be a problem at all with a 1" threshold between those bedrooms and the other flooring that you will choose. I have more height difference than that from my old fashioned mud bed set slate entryway that is open 6' to the adjacent carpeted family room, and using a marble threshold to span the difference works just fine. Never had a guest stumble yet.

Fluorescents are out of fashion now, but you will not find anything that provides better, more even lighting at a comparable cost. In your place, I would rebuild the lights with modern electronic ballasts and high-CRI lamps. But then I'm more concerned with practicality and economy than fashion.

No, level 4 is not "better" than level 1. It will filter smaller particles at the expense of higher pressure drop. You use the lowest level that will remove the particulates in your water. Using a filter that is too fine will just impede water flow for no reason. Typically, 25 micron is a good place to start. 5 micron is really small, so avoid it unless it is absolutely necessary. The filter should be upstream from your softener. If it is downstream, it is doing you no good -- don't bother placing a filter cartridge in it.