Ouch, sorry to hear about your experience.
Unfortunately "Volvo mechanic" does not necessarily mean they are expert with the D24 family of engines, which are an Audi/VW product. Here in the US, supposedly well experienced specialist "Volvo mechanics" have been responsible for the routine destruction of D24 engines ever since they were first introduced, and then they spread bad rumors about the engines after there are problems, rather than acknowledging their own ignorance and blame.
It's an easy job for a guy with the right knowledge and equipment, as you know now, but proves to be impossible for someone who insists on convincing themselves that it's just like any other timing belt, where you just "line up the marks" and there's no reason any special tools or procedures should be needed -- the suggestion being that factory just came up with all those things for their own idle entertainment, and not for any good practical reason.
Anyway, sounds like this is not news to you, having learned the expensive lesson now.
The bolt you are referring to is probably the long skinny bolt that goes through the idler and the oil pump, and threads into the block. So, unfortunately, if the threads are damaged, then the damage is in the block itself. This is probably a result of over-tightening the fragile small thread bolts by a ham-handed mechanic, likely more damage from the first fellow you took it to sadly, as he was probably unaware that the torque spec on this delicate bolt is very low.
The proper repair would be to install a heli-coil or other kind of thread repair in the damaged hole in the block, but as you note that would be very difficult to do because of the need to remove the oil pump to get access, which involves tearing down the entire front of the engine (and pulling down the oil pan as well, which is labor intensive). There is also significant risk involved in the process of installing the helicoil because this particular bolt hole goes all the way into the crankcase, which is why you see oil leaking out now. That means metal debris from drilling and tapping the hole for the helicoil could make its way inside the engine and cause damage to the oil pump and other parts. You would need to exercise great caution to prevent this from happening.
Oil in any amount on the belt will shorten its life and threaten the engine. How much will it shorten the life of the belt? -- who knows. You'd have to use judgment since there is no way to predict.
I think now that you have gone to the work of repairing the damaged valve and getting new belts on all around, you probably ought to go the final mile and get the damaged block threads fixed properly. It might involve removing the engine -- chance to put in a new clutch too. Then you'll have a fully refreshed 940 TD.
I think trying to live with the oil-soaked belt is probably a bad idea, both bad for the car and also bad for your stress level, never knowing how much or how little you should be worrying about it.