Here's how things work jnefan
A) You would need to remove your partition on your HDD and update to 3.55, yes you are right Sony has removed this feature from 3.21 and onwards, however we have got it back through CFW 3.55
B) No you will not be able to access your partition later simply because the PS3 will not read it, best you backup and format and upgrade your firmware before hand that is before upgrading to 3.55 OFW
C) I'm not a 100% sure on the last simply because i don't want you to go the wrong route and end up messing up what you have back up your stuff on 3.15 and upgrade still not sure on this though.
You will have the ability to gain Linux back with a bit more variety and look up OtherOS you will get to install linux on your 3.55 console.
Hope this was somewhat helpful to you sorry if i was not as clear or if you didn't get what you were looking for.
Side note, upgrade to 3.55 via USB and not via internet
because you will be installing OFW 4.30 and that is a no go, for OtherOs or CFW.