[*]Will I render the console unbootable if the 8th SPE doesn't work?
Yes, if the console tries to use it during boot. The probability that it doesn't work depends on the die process of the CPU. 45nm ones have a higher chance of it to work than 90nm ones.
[*]Can I alter the console so that it boots again?
No, at least not until someone finds a way to write into syscon via hardware flasher (if that's even possible).
[*]How do I enable the 8th SPE
Change value in syscon at offset 0x48C30 to 0x07 using ps3dm_utils.
PS3 Slim CECH-3004A 160GB
PS2 Slim SCPH-70004 - FMCB 1.8b