I don't know the answer on weed seed being on the test results. Rick posted a couple sample reports in another thread sometime in late summer. I agree with him that most weed seeds will be cooked beyond sterile by the composting process. The place I get it I have always assumed some weed seeds were coming in but hoped that I could out compete them. I haven't ever noticed much of a problem at all.
I'm sure handling bags is work as is handling bulk. Honestly I don't know which would be more. I used a tractor to load the topdresser most of the time. I end up doing plenty of shoveling one way or another though. It is fairly easy to handle.
What is important is to have anything you buy be screened. The place I get it has big machinery for this and will double screen it if I ask (and pay a little more). I would have reservations about what may come out of a county run composting operation. It would be worth exploring but you need more assurances. I'm sure you have already begun looking into what they turn out.
You are exactly right about it being a premium service and worthwhile at that. Part of what I was cautioning you about is doing the leg work and customer education part well enough to get this launched then effectively making a market for someone else to come in behind you and do the work cheaper simply by buying the compost at a fraction of the cost.
There would be nothing wrong with starting off with using bagged material. You may tire of it after a few big jobs (or not) and that is fine too. It is easy to switch if you have a good supplier. You know your market and yourself better than I do.