A) The D12 is still available should you want one. It won't come from DTV but it is available.
B) The H24 (or HR24 if you like a DVR) will work just the same as a D12 after you make some settings changes in the "satellite setup" menu.
I find it highly unlikely that the D12 will not let you update the settings but I am not there so I will just have to go with the information you provide. It's more likely that the antenna you are connected to is not letting you do the necessary changes as the default setting for a D12 is 18" round / multiswitch.
What I post is my 2 cents and nothing more. Please don't read anything into my post that's not there. If you disagree, that's OK.
Can't we all just get along?