Re: #3, tradeoffs of lucid dreaming,

I don't know of any studies to support this, nor have I done sufficient systematic investigation of the phenomenon, but I am a lucid dreamer, and I have noted a general trend of increased alertness and improved memory of dreams after lucid dreams. I'm not well-versed enough in the science of dreaming to propose any credible explanation for this, nor do I have evidence that memory in general is improved. However, it has been consistently true that I have better memories of lucid dreams than non-lucid ones, and I am more likely to wake up fully alert after a lucid dream than is normal for me.

I know anecdotal evidence may not carry a great deal of weight, but I hope this is helpful.

Those are some pretty challenging questions. #1 just requires some research into sleep tables and aggregate data, which while tedious is not necessarily difficult. #2 is worth looking into as a way to justify interest in sleep specifically.

But #3 seems impossible to answer now, since there are few enough lucid dreaming studies that likely none of them have investigated it, and I would expect any effect size to be small (which means the existing studies which use no ns <~30, for the obvious reason of it being hard to find lucid dreamers, will be badly underpowered to discover it) since lucid dreaming is fundamentally rare and dreams short in duration. It'd be like asking whether getting 30 minutes less sleep is bad for your memory: possibly, but you're going to need an awful lot of data to spot the ill effects!

Question #4 is somewhat similar but may be answerable for specific values.