>>1605699See
>>1605701If you feel comfortable and ride without issue, it doesn't really matter. I'd say it matter less with a mtb since you're moving around all the time.
Look at this dude. He's obviously a seasoned rider, and he rides a too-big frame out of conscious choice. He even chopped his frame to make it fit him properly.
https://m.youtube.com/watch?v=E3Qb97HHj1QI'm not saying you should do that, but you are going to be ok if your frame is a little too big.
Also to answer your question if your seatpost is alu I bet it will be fine backwards. But keep an eye on it to make sure it's not bending or anything dumb. No reason to chuck the current bike if you ride it and it works.
