Thanks for responding. First of all, is 6.0 the same as 5.10?

As for the secret level boss issue - yes, this makes a lot of sense. As a matter of fact what you described is exactly what happened to me with these kinds of levels, it's just that it's been so long that I forgot about this nuance and only remembered the level "resetting".
When it comes to gems I'll admit it's mostly a personal thing. I just like the idea of gem collecting having a purpose beyond increasing the score (not to mention the prospect of restoring a feature that was supposed to be there to begin with). Granted, in the main single player campaign lives aren't exactly hard to come by and I know a lot of people consider lives to be an outdated form of gameplay design, but I always liked the way they provided incentive to collect items and explore the level even if I don't die all that often. Regardless I think it should be optional just in case.
Thanks for taking my feedback!