I have the same model, and have installed the LG Ultra Slim Bd DVDRW 6X SATA 9.5mm Tray Retail Optical Drive BU40N and it works great.

I have also installed the EVO970 M.2, and 3 additional 4GB hard drives.

To keep things cool, I added the R5 Front Case Fan, and replaced the top 92MM exhaust fan with a the 120mm 149cfm fan that's provided with higher end graphics cards.

I'd agree that swapping the CPU offers almost nothing at this point, and would be a waste of money.

Starting with the first question...Yes and the following forum post will help.

https://www.dell.com/community/XPS-Desktops/Internal-Blu-Ray-Burner-for-XPS-8930/td-p/5857205

Yes the CPU can be removed if it fails and yes you have pretty much the highest CPU available. I would not worry too much about the CPU failing as the CPU is one of the most reliable parts of a computer.

As for hard drives you can have up to 3 x 3.5" drive with a maximum capacity of 4 GB each.

And you can have an M.2 SSD with a maximum capacity of 1 TB.

3: Can I have more than one HDD? If so how many? Is there a limit to how much storage is on the HDD?

Yes, you can have more than one HDD. There are 4 SATA ports so you can have 4 HDDs if you give up the SATA port used for the DVD drive. The specs for the XPS 8930 say the limit is 4 TB.

Actually, to Vic's contribution, in addition to the 4 SATA ports, you can count the SSD M2.0 as well.  I believe you can get up to 2.0GB perhaps more, so that's worth counting.  Again, with the Blueray burner, I have 3 - 4GB drives plus the SSD as C.   Then I also have a couple of Router hosted network drives as well.  Trust me, you will never lack with storage options with this unit.