LLB

See, if you want the degree just as an ornament, then you may probably choose any correspondence course to go for. Refer to the link below for detailed information:
https://goo.gl/1F7TuN

The specialisation depends on your area of interest. Not only civil and criminal litigations, but there are many more specializations available these days. They vary from ADR, Corporate law, Tax laws, Cyber laws and so on. Top class universities provide specialisation in these fields like Symbiosis International University, Christ University etc. Under 3 year LL.B. Programme.

No, you cannot practice with a Diploma. You need to have a LLB Degree in order to practice. Most places, including private and government require you to have a LLB Degree. However, certain paralegal work or research based work may be hiring people Diploma holders. But, full time lawyers need to have a degree of LLB.

Any CLAT comprehensive book will do.

Once you complete B.Com, you can surely appear for CLAT. You may refer to the link below for detailed information:
https://www.shiksha.com/law/exams/clat

You can surely take admission in distance courses, but regular ones won't be available for you.

PU conducts law entrance exam for LLB. It will be conducted on 17th June 2018 and application will be available by the first week of May 2018, and exam will be conducted offline only.