Keep it simple and do one side at a time, unless you have a good reason to remove the subframe.
It took me half a day to just get the carpet out of the way, so the whole job takes a few more hours. It really depends on how easily your bolts come out. With mine, I had to heat the nut contraption that you see through the hole in the firewall, so it became a longer job. But I live where rust is king and everything is difficult.
If you want to grind down the metal in the area of the repair, that is up to you. I concentrated on making my replacement piece of metal with the hole in it for the subframe bolt, and squeezing it between the subframe and the unibody. You can get the sides to curl up and touch what is left of your original structure and weld it in place along the sides. You can shine that section up to make a good weld connection.
Depending on how your floor looks in the general vicinity of the frame hat section shape, you can add a strap of metal on the top (under the carpet) in that area to strenghten the area if you are nervous of the the strength of your repair. I did not do that in my case, but that is up to you, when you see the condition when you pull the carpet up.
The whole job doesnot take long once the carpet is out, the new piece is shaped, and you have the subframe bolts out.