As a northerner, there is something called ice wine using frozen/ ie very late picked grape. It could hang two or more months. ,,,, If the flavor is OK no reason not to try them. I would be concerned about mold and any mold flavor.
Raisining is traditional as an ingredient which basically means allow the fruit to dehydrate with enough air flow so that mold has not grown. Here again hanging fruit should have good air flow so that it did not mold.
From a health point we pick grapes at 30% plus sugar which is quite stable. Ripe grapes start close to where the water activity will prevent all microbial growth, (at about 50% sugar). Another up side for health concerns is grapes are a low pH food so that food poisoning organisms don’t reproduce.
In normal processes freewater promotes infection, and dehydrated foods are stable. ,,, typical examples to be concerned about involve free water for example you are in a rainy climate like Portland with rain/ condensing humidity day after day ,,, or the food is in a plastic bag for days which traps humidity which condenses/ drips to the bottom and dilutes the sugars,, or the food has been on the ground which again tends to collect free water.
I got 2.5 L of juice. I will get a hydrometer to get a reading on this. However, assuming I did not lose much sugar. It could have a density of 208/L. Since I added 2.5cups of sugar. Not counting sugar already on grapes
2.5x 208g/2.5 L= 208g/L. PA= 9.9%
I know the old shoulda, coulda, woulda comments are tiring but as someone mentioned - if those grapes had remained frozen... ice wine is supposed to be excellent and you had what looked like the potential there. And by the way Don't look up the prices for ice wine - WOW !! They'll take and arm, a leg and your first born for bottle of that stuff.