Temporarily disconnect the cold start injector and fit a Neoprene hose to a pressure gauge over the nipple.

Adjust the regulator for 28 psi.

"...hoses leading to the injectors keep leaking and a newer injector from my now parked 1800ES is actually leaking from the plastic of the injector..."

No doubt the 30+ year old hoses are dried and rotted.

That was a common problem with those components. Back in the early-mid '70s, Mother Volvo had a recall -- you could get 'em all replaced for free (and I did on several cars, including my '72 142E).

I doubt they'd honor that recall today (joking, joking). But you can grind off the crimp ferrule and remove the defective hose. Install a new 2" FI hose, clamp at both ends.
--
Don Foster (near Cape Cod, MA)