University of Mumbai [MU]

Yes, it is possible to do a Diploma in travel and tourism. You can refer the following link to find the detailed information about some of the good institutes for Diploma in travel and tourism.
https://www.shiksha.com/hospitality-travel/travel-tourism/colleges/diploma-courses-mumbai-all

You can get admission in college but as you're not sure about the compartment exam of Maths, that's just your thinking. Just hope for the best. You're going to clear it. All the best.

Graphic Era University is the best option but if you want to take admission in any government college by state counselling, your rank should be under 5 Lacs and 12th percentage needs to be good. Then only, you can take admission in any government college by state counselling. For more details, post a comment.

Here is the list of colleges which are offering nutrition courses in Mumbai:
University Of Mumbai
School Of Ayurveda
Netaji Shubhash Institute

You can follow the link predictor for better results:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor