Using multiple adverbs for the same verb

Firstly: Is it appropriate to use multiple adverbs in a row? Yes, it is. Normally I would expect them to be separated by a comma or "and". For example:

I slowly, stealthily, went downstairs

I went downstairs slowly and stealthily

Notice that these adverbs both behave prototypically: they (separately) modify the verb.

Secondly, Are "fully" and "electrically" adverbs, despite "powered" being an adjective? Yes, I agree with your understanding, that adjectives, including participial adjectives, can be modified by suitable adverbs.

Thirdly, the actual example given: "This car is powered fully electrically". Well, it does sound rather odd worded like that. I think it is because "fully" refers, in this case, to "electrically powered". (It would mean something different to say that a car is both "fully powered" and "electrically powered".) Normally you could have the adverb before or after the adjective. So it sounds right to my ear to have "This car is fully electrically powered" or "This car is fully powered electrically", but not the phrase that you came up with.

No, your example sounds very peculiar, and I think you have hit on the reason why: "fully" and "electrically" are both adverbs of the same type occurring in the same clause. See this previous question and my answer to it.

As Peter Shor and others point out, it should be possible to interpret "fully" as a modifier of "electrical(ly)", one would think, but for some reason which I don't understand, that is not possible here, for me at least.

In this case "fully" is not needed as "electrically" already describes powered. But if you really need "fully" in the sentence, i'd rather say, "This car is fully powered by electric." instead. The only time to say "fully electrically" is when someone believes that the car is powered both by gas and electric.