10 questions.... perhaps you want to get a friend involved who is computer savy if you have never changed out a processor, upgraded BIOS, etc. Some of this is very basic, and some quite complex.
1. Almost zero issues over the years, and if I got a bad processor they sent me a replacement on the honor system approach.... plus having a high eBay score. It is a trust based recycling system.
2. Use google for xw8400 drivers and the HP link at the top will have the BIOS SoftPaq available. But, you're running linux so you probably want to have a Windows friend run that on their workstation and get the .bin file from the DOS folder in the unpacked SWSETUP folder on the root level of their C drive. Then, learn how to update BIOS from within BIOS in the forum here. You don't need to be running an OS to update BIOS if you have that .bin file on the top level of a USB drive and know how to get into BIOS. You need to read up on this all.... it is technical.
3. At least in Windows it just runs with the new processor in place.... maybe one auto-restart.
4. Yes. I've posted in here on the correct Torx long screwdriver to use, and high caution is needed when changing out processors so you don't bend the tiny contact pins in the process. All that is posted about in the forum. You can get away with using a flat bladed long screwdriver for those 4 screws but be very cautious. Those can slip much more easily than Torx. Do you know how to use thermal compound? That is in here too.
5. It is there on the motherboard, easy to find. It looks just like the first one that is plugged into already.
6. Yes. They are spring loaded so do not unscrew the fist 4 beyond when they spring back up about 1/8".... if you over unscrew they can fly out into the room and be hard to find. Don't ask how I know.
7. Yes. I know nothing about linux so you are fully on your own about how to do that.
8. The PM system here is findable under my link. I only need your email to send you the tiny setup .txt file if you want it. You need to learn how to capture your current BIOS settings onto a thumb drive from within BIOS. Read up on Replicated Setup in this forum. It is no value to send you anything unless you can prove to yourself you know how to do that because you use the same basics to load mine.
9a. Faster boot times, optimized performance for SSD/AHCI based devices.
9b. Don't know what your settings are, but mine are optimized for doing just what you are wanting to do. There can be ways in BIOS to turn off access to multiple processors but usually that is not set. Things will work quite well if you simply set things to default.
10. Airflow deposits dust. Dust insulates from cooling, so you want to take the opportunity to clear that out. It does not look so bad.....
Please understand that you need to take over this project, hopefully with a tech savy friend. Lots of info is easily available to you here by using the search box above to the left which targets this forum only.