That should work for the time being, but I think Directv figures for 7 tuners in an HR54 since it will use that many once UHD is fully implemented. I'm sure one of the installers will update my post soon enough.
Yes, that should work perfectly on a RB LNB. The 7 tuners doesn't exist as far as we're concerned and if I remember correctly the extra 2 are reverse band only so they won't count against the 13 tuners on the LNB.
I assume you mean C61K, there is also a C61 which is not 4k.