How will I know if my portable hotspot device is working with the prepaid sim card?

Yes, of course; unlocked means you can use any SIM, including the carrier that the phone was originally locked to.

It depends on the gas station. Some will not let you use a credit card to buy a prepaid card.

Yes, as prepaid credit cards act like regular credit cards.

You are basically transferring cash from a "permanent" card to a temporary, disposable one.

By inserting it into your phone. It should be automatically activated upon usage.

Dial the number given to you by the provider.

With the prepaid card, you should be able to watch Netflix on any device without a credit card for as long as the card can pay for it, yes.

Perhaps her Sim save/Sims game has corrupted somewhere and the game is preventing any further errors to happen. She should try to backup her Sims file by: 1. Going to...

Yes. Most SIM cards bought in Europe offer Europe-wide connection.

Yes, they are the same. Rewards cards are just because you spent money in a store, while gift cards are something that people give you.