"You saw me"
I take it back. its both singular and plural. So "you" is the right way. Waliniona is "They"
No. They saw me would be waliniona.
M- is also you. It's plural you, you guys, you lot, youse, y'all, ye etc.
It is not wrong. Report it should you see it again.
My standard operating procedure is that if the sentence asks you to translate a sentence into Swahili where "you" is the subject, and there is nothing in the sentence to suggest whether or not plural or singular is required, report whichever version comes up as wrong.