B.Sc

After clearing Bachelor's degree, you can pursue the master degree. If you want to get admission in government colleges, you have to appear for the national level exam like GATE. Some universities conduct their own entrance test. The Union Public Service Commission (UPSC) conducts Geologist exam for Central Government Agencies.
Top job profiles after B.Sc. in geology:
Volcanologists
Petroleum Geologists
Environmental Geologists
Earth Science Teachers
Glacial or Quaternary Geologists
Structural Geologists
Hydrogeologists
Engineering Geologists
Paleontologist

Please visit the college website and download their brochure which should contain all important details regarding admissions for B.Sc course in IT at St. Xavier's College. I am unable to provide you with the link as you have not specified which St. Xavier's you are talking about, as there are many. Different institutes may have different criteria.

Documents required to apply for lateral entry are most likely decided by the institute from where you are leaving. Normally a transfer student would require a transfer certificate, pass certificates for previous semesters and approval form the former college. I have provided you a link of Calicut University. You could download their brochure which contains information regarding admissions and lateral entry permitted by the college:
https://www.shiksha.com/university/university-of-calicut-51534

That would depend entirely on the person interviewing you for a position in the company, or placement criteria in your college. Marks do matter and individuals with a good academic record are preferred over people having bad grades, however I doubt recruiters will go as back as your HSC marks. Try and improve your marks in B.Sc degree which you are currently studying to such a level which would impress recruiters, and try not to think about the past as much.

B.Sc graduates have a wide variety of options available in terms of topics, subjects and field. They have an option to pursue a post-graduation degree in their respective field. Students can also join non-science master degree courses such as animation, management, journalism, computer technology, etc. after the completion of B.Sc degree.

Basic eligibility for a candidate to pursue a B.Sc. degree in maths through distance mode is completion of 10+2 with major in science. Please refer to the link provided below for a list of colleges which offer B.Sc courses via distance mode in India:
Link: https://www.shiksha.com/science/colleges/distance-correspondence-b-sc-in-mathematics-colleges-india

Starting salary for B.Sc. graduates in interior designing depends on a number of different factors. Prominent factors are- profile of the employer, employee skills, aptitude and grades attained by the employee. In the beginning, interior designers may earn 10-20 k Rupees per month. Salary will rise after gaining work experience. If one decides to remain self-employed, salary figure will be quite different than the ones mentioned above.