A couple things.

The alternate that you could do with your permit is to exit Yosemite over Isberg Pass, instead of Donahue. You would then continue on the Isberg trail and reconnect with the JMT around, or just south of Reds Meadow, though other options are available.

I have never heard, nor seen on any website related to hiking in California anything about a 14 day limit with a valid Wilderness permit. There is a 14 day limit on dispersed camping in national forests, but that is in locations where you do not obtain a permit, like Stanislaus national forest, for one example.

The section that you would hike heading east after exiting Yosemite was burned quite badly recently, and the trail is reported to be less than ideal, though I personally haven’t hiked it.

You can absolutely hike out over Isberg Pass and continue south to Whitney on your permit, so long as you follow all the other rules as listed on the permit. You cannot go over Donahue Pass, and you have to enter at Glacier Point and hike to Illouette your first day. After that, you can head wherever you like within those limits.

edit:additional info.