B.Tech

No, you can't get admission in the colleges which only accept JEE Mains marks. You can get colleges of your state, maybe even government ones. Try JEE college predictor to get more information and insight about college which you can get on the basis of your rank-
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor?utm_source=connect&utm_medium=CR_JEECP&utm_campaign=4067795.

You can apply for the university who takes their own exam for the admission.
Examples includes VITEEE, SRM, BITS, AMUEEE , KITEEE, WBJEE, etc.
Best of luck

Check your Chances here at Shiksha's JEE Mains college Predictor. Link is given below:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor?utm_source=connect&utm_medium=CR_JEECP&utm_campaign=6540165.

Germany is the best. MS is one of the biggest benefit for free of cost education and placement in Germany. Internships are good.

Yes, you can go there. Many of my friends are also studying in CEC Landran. It's a good college for doing CSE.

Please check for your eligibility to pursue a Post Graduate Diploma in Hospital and Health Management at Institute of Health Management Research from the criteria mentioned at the link provided below. You can also check for admission procedure followed to secure a seat in this course at this link. Regards.
https://www.shiksha.com/mba/course/post-graduate-diploma-in-hospital-and-health-management-institute-of-health-management-research-electronic-city-bangalore-277900

Please check for your eligibility to pursue an Aircraft Maintenance Engineering (Mechanical) course at Indian Aerospace and Engineering, Sha-Shib Group from the criteria mentioned at the link provided below.
https://www.shiksha.com/aviation/course/aircraft-maintenance-engineering-mechanical-indian-aerospace-and-engineering-sha-shib-group-navi-mumbai-92419
Admission will be based on the marks obtained in Sha-Shib Aptitude Test (SSAT) and medical test conducted by Sha- Ship group. Interested candidates can apply online/offline with required information. Regards.

B.Tech and M.Tech from the same branch is the best option because in that, you have a vast experience in that field that would be beneficial in every form.