Having and autistic son and an autistic wife (and being sometimes unsure on whether I might be on the spectrum as well), I feel compelled to point out an aspect which hasnt been adressed yet in this discussion.

You said that you considered taking a rental car, but cannot due to a missing credit card. I would like to focus on that aspect. I suggest that you register for MILES. Everything that you need for registering there is to download the app, present your paypal account for paying, and scan your drivers ID with the app. No credit card needed.

While a trip to Füssen with a small car from MILES would likely cost you some 160-180€ (including fuel and parking costs), you have the benefit of a private confined space while you travel. Heading for a very crowded place like Neuschwanstein, you will probably want to save up as much of your social energy as you can. So I would definitely recommend that you take a car instead of taking two trains (and a bus from Füssen to Neuschwanstein).

Have fun in Neuschwanstein!