Hi. Maybe I can help? I have a Lendrum. I really like my wheel. Yeah, the no instructions did make it a challenge to put together.
I use the largest whorl on the standard flyer a whole lot. If I made thinner yarn that needed more twist I would just move the drive band down to a smaller whorl so it would turn faster with less treddling. You could try using a smaller whorl so your foot/feet dont' have to go so fast.
I don't have a plying flywheel and really don't know what that is. I'm relatively new to spinning too, but I really enjoy it and have been trying out lots of stuff and different fibers.
I've tried several different plying methods. I think the easiest is to spin onto 2 or 3 bobbins (for 2 or 3 ply yarn) then put those on a lazy kate and ply from that.
I don't really like having short bits of single ply left over so I often use navajo plying to make a 3 ply yarn. It's like a crochet chain with really big loops that are twisted. Or andean plying to make a 2 ply. You take both ends of the single that you spun and ply them together. A wooden tool or ballwinder work well for this.
Plying really shouldn't be that different from spinning, you're just turning the wheel the opposite direction.
Test your wheel without yarn on it. Treddle one way then the other. Does the flyer turn at the same speed both directions? I'm not sure what would cause it to turn at different speeds... Does the drive band slip at all?
It should be smooth and quiet.
You could try spinning a small bit of yarn in the direction opposite what you normally spin ( the direction you ply) and see if you have to treadle fast when spinning that sample.
I hope that helps. If you still can't figure out the problem I suggest having your wheel looked at professionaly.
Please let me know if I'm on the right track at all. If you have more questions I'll do my best to answer them.