Say what?

I woke up this morning wondering about sayings that people use.  Although I have not quite woken up and haven't had my first coffee, I am trying to work out if maybe everyone else is right and I am seriously in need of a mental health assessment.

For example, one I hear regularly is "Cloud cuckoo land"........  Is that not a mix up of two sayings?  And if it is don't the two sayings mean totally different things?  Being "in cuckoo land" means going a little la la yes? and being "on cloud nine" means being deliriously happy?  So does "cloud cuckoo land" mean being deliriously happily mad?..... hummm think I might quite like to visit that land.

Then the saying "that went down like a lead balloon", I believe is an expression used when maybe something does not go down well?  But wouldn't a lead balloon go down really well?  And of course I understand the concept behind this one; a lead balloon wouldn't work.  But, hypothetically, if a balloon went up and then turned into lead at a certain altitude it would come back down really efficiently.

"Over the hill", "it's all downhill from here"......... are these used to express the current situation is only going to get worse?  But surely going downhill is much easier than up so things can only get better, no?  Maybe that is wishful thinking on my part as I see my 40th on the horizon.

But then a population of people using these expressions can't be wrong so maybe I must accept that I am "thinking too much about it" and sit gazing out of the window daydreaming about cloud cuckoo land and the best way to get there.