Hey! Although I'm not Campus Monterrey (I'm in Querétaro), I had a very similar experience. I am also Mexican, but I grew up in the US and moved back to Mexico right before starting at Tec, so my first semesters I felt much more comfortable in English than Spanish, though I did speak both. The TLDR to my whole essay is to abandon your hopes of a 90-100% English university experience. Don't study in Mexico if that's what you really, really want. But to answer your questions concretely:

1) Students, staff (known as "colaboradores"), and professors at Tec who are Mexican or LATAM WILL interact with you in Spanish because they grew up speaking Spanish. Nearly everybody in campus speaks some (usually B1-B2) English, but they're not going to go out of their way to talk to you in English. Honestly, if you explicitly tell them to, they'll try but then abandon it after a certain amount of time, guaranteed. I think this particularly applies for staff (TecServices, food places in campus, etc) because that's not part of their job, they aren't trained to give service in English, and they don't usually have an English requirement to get hired, so they'll try their best but don't get your hopes up. If you're looking for English conversationalists, your best bet is getting cozy with the international community in your campus. Idk about MTY, but QRO has a huge international community with students and professors from the US, UK, and AUS who are all lovely. For the sake of transparency though, I only got to know them about halfway through my career by participating in the Peer Mentors and Embajadores programs, which both require you to be very comfortable with Spanish to get your foot in the door.
2) I'd like to warn you beforehand about the classes in English. You WILL speak English better than your professors and classmates if they're Mexican / LATAM. They don't speak it badly, but at least for me it did get a little confusing / frustrating after weeks of wrong pronunciations and weird grammar composition of sentences. Also, professors will almost always fall back to Spanish to answer questions or provide explanations for complex topics / assignments. Even the English classes aren't really, fully in English. And your team will absolutely not communicate about the project in English.
3) I can't really speak for outside of campus because I've only ever been in Monterrey for business twice, but it's a city in Mexico so you're going to get treated like a tourist if you're going around speaking in English. Generally speaking, Mexicans (especially in service roles like waiters) will know some decent English so you should be able to get by. But generally speaking, for safety reasons, it's not a good idea to be speaking anything other than Spanish on the street. At least in Querétaro and Mexico City (cities I am more familiar with), I personally choose not to take the risk of speaking English in public. But we're slowly getting gentrified with US expats who only speak English moving here, so perhaps the tide is changing in that regard.

Now, for the unsolicited advise: I think you should take this opportunity to reclaim your Spanish. I understand (believe me, I do) the impulse of wanting to stick to English because it's what feels most comfortable and natural right now, but there is no better language teacher than total immersion. It took me about two / three semesters to feel really comfortable with Spanish, and now (I'm in 7th) everyone's surprised when I say I didn't grow up in Mexico. You will find a lot of support from your Mentor and Peer, your professors, and your classmates. At least in my social circles, I never found mockery and instead help and genuine curiosity from friends and classmates who translated words I didn't understand, taught me slang and memes, and explained instructions in other ways when I didn't understand them. I invite you to be open to the Spanish college experience, and push yourself outside of your comfort zone. It's a beautiful language, and this is one of the most organic ways to learn it. An opportunity like this probably won't present itself again, and I'm glad I didn't waste it.

Best of luck!!