Flooring – Level hardwood floor that was covered by baseboard

If you look at the underside of the tub (through your proposed hole) you should be able to tell if it was meant to sit on the subfloor or not. Some kind of wooden frame will be partly embedded into the fiberglass to help support it. If there are "skids" that should touch the subfloor but do not you can use shims to support them.

I had a shower enclosure that was not well supported underneath. It eventually cracked and leaked on the ceiling below. When I removed it, it was obvious that the enclosure was sitting on the floor in the front and hanging from the wall in the back. The reinforcements on the bottom of the enclosure were not touching the floor. I don't know if they were meant to or not.

Most installs I have done we have trimmed around the fireplace. There are several ways to do it. If your fireplace hearth sits on flat ground we try to meet even - which is almost impossible unless you did the floor and hearth all in one install.

Most of the time we trim around the fireplace first and then cut to the trim. This is very very very time consuming because you will potentially have a lot of boards meet the trim and they need to be within 1/32".

An easier way to do this, especially if the hearth material is not as even, is to add a riser then maybe an optional board (or partial board). So you would layout your wood and just get as close as you can to fireplace then nail down the riser around the fireplace. The corners take some work but this is pretty easy. This looks really good if it is brick and you can get (or notch) the riser so its back sits in a mortar line.

And a note on floating - I would probably use a riser. Because you could set the floating wood 1/4 inch from fireplace, giving it a proper gap, and then staple the riser just to the underlying floating wood. It would still be floating then.