I have some trim on the fire place one problem that I found out after I started is the fireplace drifts about 3/4" so where the floor meets it at one end there's a 1/4" gap and at the other it's got almost an inch( I measured it off of the wall and the floor is square with it), I did fill the gap with some dark colored caulking to hide it. there is a chair over the larger gap to hide it now also, as for going back and under cutting I wouldn't know where to start especially with you the tongue and grove are laid out where I would have to pop it into the under cut since I started off the wall.
Sorry to hijack but a few YouTube videos can go a long way.
Sorry to hijack but a few YouTube videos can go a long way.