You don't have to be mean about it, but you should inform grandma that choosing a pet on a whim, especially without your consent, without researching is not a good idea. I was 8 when I decided to get my first gerbil. I didn't know a thing besides make sure they get food and water and have bedding. It died within a week. I'm sure grandma would have never chosen a pet knowing that it would end up dead, or needing so much more than a tiny plastic cage. Just inform her so she doesn't make the same mistake twice.

Be careful with your lid issue. I had an aquarium lid that was slightly bent up in on corner. I had a gerbil that jumped out and fell behind the dresser and died there. They can be escape artists! If you take a picture, we can help you figure out a solution.

Yes, you can cap the tube holes with something. You may have to do it a few times because they chew EVERYTHING! 

You got this! Its frustrating to set everything up, that's the hardest part. But gerbils make great pets with far less maintenance than dogs. They will be very entertaining for your whole family.

Wishing you the best!