Whether you are overclocking or not, using both 8 pin EPS connectors would be a good idea. Depending on the board, it may not even work without it as I've seen a few instances here where users had to replace their PSU with a model that DID have dual EPS connectors because the system would not POST without it.
So, you can TRY it, but it might not even be an option regardless of overclocking considerations. Typically they SAY that it's only if you are seriously overclocking but that has not been the case in at least SOME cases.
And whatever you do, using any kind of "adapter" is an extremely BAAAAD idea. If the PSU doesn't HAVE a second EPS connector built into it, then it is not suitable for use with that platform.
Yes. 4.5Ghz would be an underclock, since the factory all core boost for the 9900k is 4.7Ghz. In reality there is little room FOR or need TO overclock, if you have ANY expectation of the CPU having a decent lifespan as it is already factory stretched-thin as it is in terms of toeing the line in regard to it's potential for electromigration and voltage-threshold shift, which are explained in the Intel temperature guide. This is a CPU that has just about been factory overclocked to as far as it can be without them simply saying "Look, this is going to last about two years if you're lucky".
Anything beyond what it's already configured for, is simply saying "I don't care if this lasts or not". The higher binned KS really doesn't extend that much either. It's still the same silicon, just a slightly better example. So, overclocking potential is practically non-existent even if you delid and even if you run a big custom loop.
If you gained 200-300mhz for your nearly four to five hundred dollar delid and loop investment, you'd be lucky, and you'd still probably cut the lifespan of your CPU by 1/3 in a lot, if not most cases.
But even at the stock configuration, this CPU can chew up some power, so I'd err to the side of caution and stability, and give it what it asks for. Obviously though, it's up to you how you want to proceed.
That's just a mid tower case, there shouldn't be any problem getting the EPS cables to reach on that. If it was a full tower I could see it, but unless there is some glaring shortcoming on the RM850x which I've never heard about, it shouldn't be an issue. It would be highly unusual if it was.