Continuity mistake: When Gordon is on the ground after being knocked down by the door, in one shot his left hand is above his head, but in the very next shot, his left hand is below his head.
Other mistake: In the final fight, V slices a man's right wrist, but blood only comes out of his left arm.
Continuity mistake: In the final fight, when V slices a man's chest twice there's no blood on his face, but in the very next shot, there is.
Answer: Nothing is ever revealed about V's history before Larkhill so we don't know why he was there. As for the roses, in the original comic book, in order to keep him compliant during his time in captivity, V was allowed to tend a small garden while his health was monitored. It was during that time that he first grew the Scarlet Carson roses and also surreptitiously obtained the chemicals that he would use to destroy Larkhill and make his escape. Whether this can be considered consistent with the film storyline is an open question - certainly nothing in the film actively appears to contradict it.
Tailkinker ★