Hello, thank you for your review and bug reports. Quick answers:
1) Warp to level 4 is anticipated (similar warp is available from level 2)
2) It was indeed a problem with that level in demo, in full versions two missing doors were added thus number of keys is balanced. There is one room with extra jewels guarded with two doors. Opening these doors is not necessary to finish the level, maybe you missed that one?
3) Yes, mask can swallow the key thus preventing finishing the level. This is a smart inventory management that has been implemented in the game, as it is not possible to throw away items. I can reconsider this mask thing, however and include this in bug fix release.
4) Double jumps has been already pointed out and will be fixed in bug fix release. Other platforms didn't have this problem and simply nobody pointed that earlier. I myself learned not to keep fire pressed but agree, this is frustrating, esp. for somebody that switches from Atari to C64 version.
5) Oversimplified collision detection is a problem, I'm trying to mitigate this by fixing the map design (not to let Tony to jump into the wall).