roskopf troubles

There is good reason for reluctance, you approach us with an admitted lack of experience and knowledge that may make helping you very difficult.
That aside, can you not move the regulator over more (away from the stud), or re-pin the hairspring at the stud to be shorter?
Are you determining this loss of time via a timing machine, or only by watching the hands move? If the latter, a loose cannon pinion (tricky to adjust on Roskopf calibers) could make you think the watch rate is slow when actually the hands are not being driven all the time.

The dial washer will have no effect on the cannon pinion slippage. In most Roskopf designs, I think the wheel mounted on the barrel contains the actual clutch mechanism that engages when setting the hands, and if that isn’t right enough, you will show a loss of time even if your rate is normal. Tightening those is not as straightforward as a traditional cannon pinion. Do you feel moderate resistance through the crown when you set the hands?
Back to the hairspring, how is the gap between the regulator pins- can it be closed any?

Read up! It would be best to understand the effect of the regulator pins in the hairspring at varying amplitude before you start adjusting them. Knowledge won’t prepare you for how delicate they are- they can be easily broken if you aren’t careful. I’d check your micrometer too, 1.5mm seems just about impossible for a regulator pin gap ;-)

A set mainspring (your’s sounds pretty close to it) will cause other problems, but not that large of a rate error spread. Going back to your original question, I think it is best to know the cause of a problem before solving it. In this case, if you have no reason to believe the hair spring and balance are not matched together, then you shouldn’t just go removing weight to solve the problem. I think I’ve exhausted my capability to help from here, it’s hard enough to fix a watch you can see!