Not the same thing at all. The optimized Dexer is a 'thing' that dexes. Dexer optimization is code manipulation to increase execution efficiency. Confusing I know.
yes, it seems once you stuff "lots" of code into an application? Then that message starts to appear.
Never noticed a speed drop when using release mode - but it would have been nice if I had "noticed" what the last change was that caused the fall back process to occur. Then perhaps I could figure out what to avoid? But, I can't say as you noted? - does not seem to be show stopper or an issue.
Optimized Dexer - makes compilation faster. Doesn't have any other effect.
Dexer optimizations - forget about it. You don't need to touch it.
You can disable the optimized dexer in the INI file. If the optimized dexer fails all the time then it makes sense to disable it. It can fail from time to time due to the state of the background compiler.
Nothing bad happens when it fails except of making compilation a bit slower.