PDF CMYK vs. Grayscale

I'm an illustrator self-publishing a coloring book to print via Amazon. The interior art is line art with a black line. I want the black to show very black I created it using iPad with a vector app that only had RGB. Then I made a PDF out of the images. I did not convert the art to CMYK before creating the PDF. Amazon says to use CMYK when you upload . 

When converting the PDF to CMYK in Acrobat, should I put Any CMYK (Amazon told me to do that, but I don't completely trust them) or Grayscale? When I do the Acrobat "Create Inventory" analysis, I'm seeing that whether I choose Grayscale or Any CMYK, it says four plates, Cyan, Magenta, Yellow, and Black. I don't quite get how the art got converted to CMYK. Am I safer using Grayscale to get the best black line? 

(just a note, I left it in RGB the first proof and it came out gray, not black--I'm only mentioning this because it tells me the line has the potential to come out gray, and I want to avoid this).

Another question, in the Acrobat Convert Color Tools where all this fun stuff resides, do I check all options of these three: (1) Preserve Black (2) Promote Gray to CMYK Black (does that get rid of any gray and make it all black? Sounds like it) and (3) Preserve CMYK Primaries--I'm thinking of not checking that one because it's not color art.

I got one proof where I had chosen Grayscale and it was black (could have been a bit more so but it wasn't bad--I had checked Preserve Black and Promote Gray to CMYK Black) but I wonder if choosing Any CMYK is a better way to go or would that make it worse.

Thanks!