Yeah, some of it is kinda intentional, in the way that when I've tried to implement a feature, it has accidentally ended up with a neat side effect that I haven't tried to (or maybe succeeded to in some cases) remove. I can't remember what my original vision was, but at the moment my strong opinion is that even though I try to be as compatible with the original engine as possible, in the end I'm already at a point that a complete clone would be difficult without revamping most of the inner structures. Their limitations would especially be detrimental if/when it is ever going to go into the territory where completely new features are added in.

I actually listed enemies walking up and down the hills as a bug in the OP (which I should clean up soon), but I might just as well keep it, at least as a default behaviour. Your observation with the coins being bounced by the platforms was correct as well: the platforms have standard collision checking with all actors. Only players can be carried by them at the moment, though – expanding that to other stuff could be something interesting to look into later.

Thanks! I'm assuming you have better knowledge of the physics than I do, so would you mind to elaborate on how tubes seem incomplete? Factors like speeds are fairly easy to change and they've seemed good enough, so I've kept them as is for now and focused elsewhere instead, but if they seem to be off significantly, I might look into them in the future. For the springs, you are probably correct – changing how it works would require some non-trivial changes at the moment, though.

Disclaimer that the above reply to Violet applies to this as well though: being a 100% clone is a very low priority if at all if it comes with a high-cost compromise somewhere else.