Couple of questions aboutg air system on class "A"

I would definitely NOT hook up the old dryer that the previous owner disconnected.

Only reason to spend the money on fittings and time to bypass is because of a failed dryer.

Yes, there are some failures that would not be harmful to reattach (like finding a massive air leak), BUT if the desiccant has denigrated, reattaching the dryer could spread desiccant powder throughout the air system.