It appears that there are two serious issues shown in the pictures. First, there is virtually no insulation in the ceiling at all. Assuming that the bottom chord (ceiling joist) of the truss is a 2x4, then there are places where the (blown fiberglass?) insulation is no more than an inch thick. Second there appears to be absolutely zero soffit venting. Given that, it's probably safe to assume that the roof lacks a proper, continuous ridge vent also.
The first goal here should be to retrofit the attic with everything a modern roof structure needs to perform effectively. First issue is ventilation. It needs to have continuous soffit and ridge venting added. The sorry- assed gable end vents need to be blocked off, they are doing little at the moment, and WILL screw up the performance of the continuous venting system. Next step is proper baffle trays to prevent blocking the air flow under the rafters. After the trays are stapled up, you take short pieces of fiberglass batt insulation and roll them up, like a small sleeping bag, and jamb them into the area where the ceiling meets the baffle tray. This seals the area, and prevents blown insulation from ending up out in the vented soffit. Finally, a new layer of blown in insulation. Cellulose is great for this application, and a level of R-50 can't hurt.
As for the mold, I would first determine if the roof sheathing suffers from minor mold issues, (as it appears to) or if it is truly saturated and unusable. The way to properly inspect the sheathing is with a sharp tool like an awl or ice pick. If it is truly rotted to the point of structural failure, you will be able to easily push and awl right through it. It will also typically have visible deflection between the trusses, and feel soft when you push up on it. If it's really shot, you will be able to easily dig through the laminations with a screwdriver, or pry bar. If it's just mold, I would hit it with a mold killing agent, in a pump sprayer, while wearing the proper protective clothing and respirator. I would use bleach, and have done so, numerous times, with great results. That said, any time you mention bleach on a forum, somebody shows up to claim it is of no value. BTW, this is a classic case of the mold remediation being a scam. Four grand for a light coating on sheathing, sorry but no. It is not in the interior of the home, it can be easily addressed as a DIY project, and the issue will resolve itself when the roof is properly renovated. I remember my insulation contractor doing battle with a mold remediation contractor that was trying to scam a homeowner. The guy tested the air in a well ventilated attic and it said was positive for mold. My insulator told him to stand in the yard and do the same test. The mold contractor tried to change the subject since he knew he had been caught. There is mold everywhere and you will find it in the air in your yard, in your house, or in a well ventilated attic.
Once the roof is functioning properly the mold issues will resolve, as the sheathing will dry out, and no longer support mold growth. Remember, especially in your location, mold is everywhere, and it will grow in the right conditions. If you remove the dampness from the sheathing with proper ventilation, you remove the right conditions for mold growth, permanently.