I selected a title from the Author challenge list, and realised that it is not capitalised as I would have done - only the first word starts with an uppercase letter, so that it looks like a sentence rather than a title. If I change this, does that affect the validity of completing it as an Author Challenge? If I leave it as is, will it be rejected (or changed by an editor)? If changing case would invalidate the challenge, I see a potential problem, as I really think it SHOULD be changed!

Should I contact an editor from the category where I plan to submit it, and alert them to the issue before submission?

In my experience, it doesn't matter. I've changed capitals and added quotation marks. I've even seen an instance where a title was changed to something else entirely, yet it still counted as an Author Challenge.

Capitalize it- while changing the title in its entirety will cause a red flag for editors (And goes against the challenge concept), issues like you're talking about are perfectly fine.

thank you linesmen, thank you ball boys. I will make my title look like a title!

It's always good to butter up the coach too - often results in getting to play first line ~

(mind you, when I'm playing indoor soccer, I tend to thank most the folks supplying the oxygen tanks LOL)