Science

Well, it depends on many things. But I observed that students having ECET rank below 100 (branch wise rank)are having good chances for lateral entry in SVUCE. There are exceptions too but as far as I'm concerned, majority of students who sought lateral entry have ECET branch ranks 100.

Yes, you can get admission in B.Sc horticulture in government college. Your marks are good enough to get admission there. you can try to get admission you may get admission.
Good luck.

The syllabus for the entrance test is pretty much high school mathematics, only the questions are much more challenging. As for preparation, you could start solving sampler papers available at the website, maybe even the sample papers of ISI Admission Test as they are more or less similar. If you still want more then you should try joining Art of Problem Solving (AoPS). AoPS could be intimidating if you aren't familiar with Olympiad mathematics, it's for me at least.

The best government colleges in India offering B.Sc in forensic science are:
Institute of Forensic Science, Nagpur-Maharashtra
Lok Nayak Jayaprakash Narayan National Institute of Criminology & Forensic Science, Delhi

Yes, physics is also included in SNU entrance exam. SNUSAT and APT do not require any sort of intense preparation if you have appeared for JEE Mains. Physics and Maths portion for engineering and natural sciences is the same as JEE Mains. Maths portion of economics is also similar to that of HSC. Rest of the courses for School of Humanities also requires logical thinking.