Fuel pumps on these Volvos are fairly easy to replace since dropping the fuel tank is not needed.

This video is kinda long and it's on a Bosch Volvo BUT it's thorough and it's essentially the same process for a Regina pump.

If you have a sedan rather than a wagon you can access the fuel pump through the trunk.

 

Auto zone...advance Auto... O'Reilly...will rent you a fuel pressure gauge BUT it can be difficult to connect it to the Schrader valve on the fuel rail.

Fcpeuro now sells a gauge with a 90degree fitting but it's kinda pricey.

https://www.fcpeuro.com/products/vol...RoCwWkQAvD_BwE

 

The fuel filter can be difficult to remove after being exposed for nearly 30 years to the weather.

Two wrenches are best used. One to hold the filter...one to loosen the fuel line.

Soaking the connections with PB blaster..liquid wrench...a 50/50 mix of acetone/ATF fluid over a period of days could help with disconnecting the fuel lines.

 

Thank you everyone for the advice. I’m going to try some things out over the course of this week and will follow with updates.

 

Well I’ve had some developments with the car. I had to move it to get out of my parking lot for repaving and was able to stop it from stalling by giving it extra acceleration. Once the car was in 2nd it was driving relatively fine. I wanted to test this further and drove the car to work today (about six miles). It did well the whole time but was a little rough. The way back home was a different story however. Stalled twice and had a hard time accelerating throughout. I had checked that rear temp sensor and everything seems to be in tact. I haven’t had time to try the fuel filter again but that is my next move. I thought the fact that the car drove relatively fine this morning and then really rough this afternoon might be telling. I was surprised I could drive it any distance at all when I first realized. Let me know what you all think. Thanks.

 

I know this isn't along th lines of fuel pressure stuff, but in your first post you mention an open spot in th pipework from th airfilter, do you mean an open hole or something? Is it after th maf sensor? if it is, so could be another factor to look at, tho this is entirely based on speculation depending on what you mean by an open spot.
Can't comment on other aspects re fuel system as mine hasn't required me learning how it works, but I know that you don't want an unmeasured air inlet between maf and throttle body.
There again, yours is prob a turbo, and as mine is a 2 litre non turbo, there may not be enough similarity to offer a proper comparison, I've got th inlet from th pcv and a pipe that loops around to what I guess is th pressure regulator or something just behind th throttle body on mine, and those are th only unions I can recall on th inlet pipe. You may have a series of small or slowly developing faults, all combining into one big symptom, that's what I found.