I tested on chrome browser and it worked fine, can you be more specific about the steps to reproduce the problem? And also, this is a wild guess but it is a behaviour, not a plugin, so you should put it into behavior folder, not plugin folder, maybe that is why?
It does work with expression to some extent; Though you need to know how to convert numerics into strings and vice versa.
I don't really understand the problem you encounter, are you using Set Target X and Set Target Y? Or are you using create/initialize tween? Sometimes using Set Target X and Set Target Y would be more convenient, depending on the case. I could help more if you could provide me with a simple capx describing what you want.
I am not sure either. If it is on the weaker device with less processing power, it might be because LiteTween is using precalculated array instead of having to calculate the lerp every tick, so it is lighter on the CPU. But if it is on an adequate PC/mobile, lerp/dt should have about the same smoothness with linear LiteTween.