There are many Midrashim that were around then that are not not around anymore. If Targum Yonasan was composed or compiled during the Rishonim period, the author drew from these same Medrashim.
Until benignuman provides a reason to assume Targum Yonasan was not written by Yonasan ben Uziel don’t assume it.
There’s definitely nothing to be wary of.
It’s very hard for us to sayy that Rashi’s Mekor had to any specific place. There are many Midrashim that were around then that are not not around anymore… We can’t bring any proof.
I beg to differ. Rashi’s makkor doesn’t definitively have to be from somewhere specific but it can be said that it is likely to be from somewhere specific.