If it's a 32/34 front chainring, and the chainline is not all to cock for some reason, I can't see why running from that to big at the back would be a bad idea at all… after all, you should be able to use the entire block while sat in middle at the front, and that is all this is…
As for peddling backwards and it changing gear… I'd expect it to be a sticking link in the chain as it shouldn't change gear if everything else is spot on. I can quite happily do it on all my bikes, including the 1×10 setup on my crosser. Does it tend to pull on the rear mech a lot when you do it?
Also, when it changes gear, does it stay in 3rd? Or does it then try and change back into 1st again? If it stays in 3rd I'd have a look at your mech and see if anything funny is going on there, and see how angled the chain is going into the top jockey wheel.