As an expat (US Citizen), can I use a relative's US mailing address to file a federal US tax return? I would ONLY do the federal return.

This is kinda hard to explain, but I'm a dual citizen -- Canada/USA -- who has never lived in the USA except when I was a toddler. However, the USA doesn't really recognize any citizenship other than its own. That means I have to file US federal taxes, even if I've never lived or worked in the USA, or have no address there. Basically, everything I do at home is technically considered an "offshore bank account" or "foreign income", and the IRS wants to know all about it... thankfully, I don't actually OWE anything, but I still have to report it all. I've done this for a few years now, either with a professional (unfortunately this is SUPER expensive) or by using the free fillable forms (the PDFs). I technically could do that again... but it's a huge mess, so I'd honestly rather not. Even if I don't owe anything, I'd rather not make mistakes. That's why I'm hoping I can use some kind of software. I'm a student, my taxes are pretty simple, and I have a low enough AGI to qualify for freefile.

The problem I'm having right now is that without a US address, most tax software won't even let me use it. (Not without awful premium fees, anyway.) However, the vast majority of my family is in the USA, and I've used my grandma's address before (with full consent) to make a US bank account + get my US passport delivered. She's happy to help and said I was free to claim it as "my" address if I wanted to. The problem is that I don't even know if I'm allowed to. I have no plans to file a state return, because I don't have a state. But would using my grandma's address as a mailing address cause anyone to get in trouble?

I'm trying to use FreeTaxUSA specifically, because the IRS website suggested it as an option... but if any other software works nicely with expats, I'm open to suggestions! Thanks for the help.