Vac canister diaphragm confirmed good. It’s connected to full manifold vac (per everything im reading, including professional recommendations, full manifold is perferred to port).
that is your problem. At idle manifold vacuum is high 15 to 16 as you pointed out.
That means the vac can is full in adding 10 to 15 degrees.
These cars were designed to have a ported vacuum source for the vacuum advance. Very low to no vacuum at idle to the can. Then the instant the throttle is opened it sees vacuum and adds advance.
You can make a timming mark. You just have to determin when the #1 piston is at TDC of the compression stroke, then make 2 marks one on the balancer and one on the front cover. Get a timing tape and you have all you need to time it properly.