If you really plan to keep this car for another 100,000 miles, like you said, I think you should go ahead and get it fixed.
Like others have said, if you can find a good independent Volvo mechanic, you will save a lot of money. The problem is that it isn't easy to find a good Volvo mechanic. Volvos have a lot of issues that are unique to Volvos. They are expensive to buy, and as you have just discovered, they are expensive to maintain and repair. Unless you are prepared to shell out serious money on a regular basis, you should trade this car in for something reliable and inexpensive to maintain, like a Honda Accord or a Toyota Camry.
I find some mechanics don't know the difference between an oil leak and oil seepage. If you only have oil seepage, you can try a "high mileage" oil that will make the seals swell slightly, and you can keep a close eye on the level of the oil, making sure it never gets too low. If you have a real oil leak, it could get worse and leave you stranded, so you would be better off just getting it fixed.