How do/will they call actual electric cars?

Duo marked のり wrong for me. I'm not entirely sure what the difference is.

Or it means to get on. As opposed to おりる, get off.

"Use" is universal for anything. Use a chair use a pencil use a car use a lamp etc

車 (しゃ, as a suffix) is more "vehicle" than car, hence 自転車, 電車, etc. The more specific word for "car" is 自動車（じどうしゃ）, so electric car is 電気自動車（でんきじどうしゃ）but it's a bit long-winded and I imagine "EV" is used more casually.

You use wo for direct objects. Ni is used for location. In this case, tsukaimasu is the verb so you are "using the train". In other cases such as norimasu, you are riding "on" the train, so you use ni. Ha/wa is typically used for the subject. Watashi wa densha ni norimasu, for example.

No, the verb in it's infinitive form is つかう. To make polite form, remove the う and add います = つかいます。

Don't bother they don't always give you the audio for all the kanji. Especially if there is an alternative hiragana option or it is a kanji they have already drilled in practice many times but sometimes they make a small error here or there. Just use your time to keep studying. And practice your kanji like crazy. Repetition is the key.

Yes, same for me