I would wait until bottling time to add sugar. The wine must be stable. Add potassium sorbate also to prevent re-fermentation.
unfermented grape juice works best when the juice used to sweeten the wine has come from the same juice that was fermented to make the wine. This results in coherent flavor. If you know you want to make a sweet wine at the start, set aside some of the grape juice for sweetening before you add yeast.
Port uses a more complicated approach during fermentation by adding high alcohol liquor 16%ish where yeast can't tolerate it.
Yes. I would calculate how much sugar you would have needed to get the original up to about 1.090 and add it. Your alcohol will be too low now with that starting sg.