I certainly don't have the expertise to give you a suggestion, but I wouldn't cut on line A from what experience I have and have observed. It would be a short panel and I am not sure how the strength would be afterwards. Besides aren't the interior end panels in your trailer fiberglass? I know the end panels in our 1963
were... (I think it was fiberglass, definitely other than aluminum) I am not positive, but in our opening like you have under your window... there might be blind rivits in there somewhere... so cutting a line on A could make it very difficult to fix. I would also check to see if in your model the belly pan comes up in there somewhere in the c channel and the bolts...
There are a number of threads where folks have replaced only a portion of their floor. I would do a search. I know I have seen them and with pictures too.
My hubby replaced our entire floor and he discusses it in various places in his thread. I will give you a link, but you will have to scroll thru the pages to find all the info.
Good luck! Hopefully someone with more expertise than I have will jump in and give their advice!
Mrs. NorCal Bambi (traveling in S Tardis ~ from the Great State of Jefferson)