The vast majority of code must be written during the jam, but you can use whatever art and audio assets you have the legal right to use.
So yes,  self-made 3D models can be (re)used.

Yes indeed, that is no problem for the gift code. You can still get the bundle and its content with the gift code.
The bundles are just not available to buy anymore.

PS: If you search the web for some bundle you probably can still find some videos or website that discuss the content. So you can see what the bundle is all about.
And you can ask for a specific bundle and I will tell what is in it of course.

Yes you can, general scripts like character controllers or input handlers may be reused in games for this Game Jam.

We always give a few hours extra for the ones that have trouble building or uploading their game , but no more than that.

I too had exams.