These are things that your realtor should be addressing with you and giving you suggestions on how to proceed, but the two options you listed are your options.

I'm worried about this because they could either reject to repair,

That's possible, but if you're at the inspection phase, it's unlikely they will straight up refuse to do something about a major issue like mold. Again, your realtor should be advising you on this and placing it in the repair order. You then need to see what the seller says.

hire the cheapest guy possible and not get it done right.

That's also possible, however you have to be provided with a receipt that the work was done, and you can ask another company that you do trust to come out and give an inspection after the work is done to verify it was done properly.

I'm worried about this option because once they get down in there, more problems might be uncovered that will be even more costly and we'll be on the hook for it.

If there were more issues, the inspector would have noted them in most cases. If you don't trust your inspector, hire a new one. Another $150 for another inspection is worth the cost when you could end up paying thousands to fix issues they didn't catch. Finding issues is literally their entire job.

I've also heard it said that mold never fully goes away.

If it's treated properly with anti-fungal spray and then painted with anti-fungal paint, it shouldn't be an issue anymore on that part of the house. You should, however, find out WHY mold is growing. Is a vent duct unhooked? Is there moisture in the area because of a high water table?

Also, realize that if you are looking at a house in an area that is reasonably wet a lot of the year (like the PNW), every house is going to likely have mold. Every. House.

We just sold ours and the inspector found mold in our attic because it was not vented properly. We lowered the price (closing costs) to compensate the buyers.