My pleasure.
Your first example is correct. The nightly is overwritten every day and then a "random" sample is taken from the daily backup to serve as the weekly backup. Same goes for monthly. So depending on what date your account was created, it has an influence on what day your weekly and monthly (if you have monthly) is archived.
Sure but you would have to write the software, we could write it, or you perhaps can find a 3rd party solution for that, but it is not built into the regular backup system and would not be a good option for shared hosting due to load and disk space requirements..... Its probably why its not built into cPanel. That means if you had a 1 GB site on the server it would actually be using up 9GB of space (1 GB in /home and 8 GB in /backup) and there would be some complications there on how to find a large enough backup disk for that sort of backup schedule, and still be affordable. Might work fine for a small site on a dedicated though.
Please explain this one I am not conceptualizing the idea.