Removing wall between living room and porch?

Hello, I am currently renovating a 1920's era 2 story cottage. The original 1st floor is sitting on the joists (has basement) but the flooring is 4 layers thick. 3 layers hardwood and 1 layer laminate. I want to rip it all up down to joists and replace with subfloor and then tile on top. One contractor that i talked to said to be very careful in removing all the layers and to leave at least the last / original one as that would be supporting the load of the interior walls. Does the flooring support interior walls? I thought that was the Joists that did that. My goal is to leave flooring next to the existing walls, not pull or rip it all out from under the sheetrock, but cut flush straight down. What type of tool allows that? Sawzall seems possible but cuts would be rough. Skill saw wont fit in that close. Maybe a multi-tool? Please give me examples if you have a solution. Thanks! MARK

so, I had a leak last year and wanted to attack it now that I have some time. I do have roof damage and need to replace some of the wood and shingles, but have somewhat repaired that so I can do the inside work now. I have cut out roughly one full sheet of the ceiling, and have replaced it with 1/2" drywall. That is what was previously used. The problem is that there is high and low spots, as well as even spots. I'm not sure why some parts sit higher or lower and it's rather frustrating. I've read about shims and all but I have already screwed the whole sheet into the ceiling. I'd say the uneven parts at the worst are a little more than 1/8th of an inch. I think some of this is due to SEVERAL layers of paint as well as the applied texture on the original drywall. Any advice on how to even things up?