I can help with the ABS/STC, brake warning problem.

All the ABS control units (Installed on 96 and newer 850, 70 series and S80 models) will eventually fail after they reach 65,000 miles or 5 years. The average is 85,000 miles and/or 6 years for failures.
The erratic ABS light and/or no trouble codes is an indication of a defective ABS unit.

The ABS/STC control module on your Volvo S80 shares a lot of information and
functions with the other computers, such as transmission and engine
management; hence, you may have multiple trouble lights flashing randomly (Check Engine, STC, Brake Failure, Warning triangle, etc.).

There are very good chances that all your problems are associated to a
defective ABS control unit.

A dealer will charge from $60.0 to $120.00 to diagnose the problem and about 95% of the cases the mechanic will recommend to replace the ABS control unit.

Only VADIS and high end OBDII scanners can read the ABS/STC trouble codes and that costs a pretty penny.

Here's more information on this issue and a replacement alternative.
Vitor's Volvo ABS website