The alternative approach is to up the ISO setrting. ISO1600 on the K-5 is a very usable setting, not sure about the K-7.

I would point out that the conclusion of the 43mm test was that the lens was Highly Recommended and that the test results did not show it was soft in the corners, but I think those who are interested should read it and make up their own minds.

As regards a choice, I would put the 77mm top of the list, followed by the 70mm, but in a way that could be the 31mm second.

It's a bit artificial to ask for a choice between these lenses though, because they are all very different. The 21mm is a million miles away from the 77mm in terms of its purpose. So how can there be a choice between them? It depends on what sort of images you want to make.

My choice is based upon what I would want a prime for, indeed what I do want a prime for. It might not be what the OP wants a prime for.

250mm is quite some telephoto, so needs a tripod, especially with the limited maximum aperture.

A few sample shots might help, but it could well be faulty or damaged if it's not performing well. It should be at least as good as your kit lens.

Firstly, the Pentax 18-250mm is quite a good lens, certainly capable of sharp images. Sue uses one of these a lot, so look at her EPZ portfolio (sueriley) to see what the lens is capable of. If yours is not sharp, something is amiss.

The SMC Pentax-DA 55-300mm would be my recommendation. See the reviews section of this site.

https://www.pentaxuser.com/review/smc-pentax-da-55-300mm-f-4-5-8-ed-lens-review-...

What about this:

http://www.amateurphotographer.co.uk/news/Pentax_releases_645D_in_redlacquer_fin...

I like it!

2 or 3, but I personally prefer 2.

It probably would.

Being a limited Edition I can see you might be torn from just returning it or having it fixed. There are probably not many silver K-5s left to swap it for.

I would talk to your supplier, but first you need to be clear what you want. Repair? Replacement? Refund?

Excellent outcome, have fun!