JoSAA

Yes, you are eligible to register for JoSAA counselling even if you are not eligible for JEE Advanced. Your 31 percentile and 30,000 rank in ST category allow you to participate in JoSAA, although the chances of getting top NITs may be low. You may have a better chance in NITs from the North-East or in less competitive branches. Make sure to list all available options during choice filling and consider state-level options too.

With a 16000 rank in JOSAA under the OBC NCL category you have a good chance of securing a seat in various National Institute of Technology Indian Institute of Information Technology and government fund date Technical Institute all the your preferred branch might required some flexibility

Yes, you have a good chance of getting a CSE-related branch in some NITs and IIITs through JOSAA counseling with an EWS rank of 68730 and a CRL of 512048. While your CRL rank is lower, your EWS rank will be considered during seat allocation, giving you a better chance at getting into your preferred programs. 

Yes, it is possible to get a B.Tech in Mechanical Engineering through JOSAA (Joint Seat Allocation Authority) or CSAB (Central Seat Allocation Board) in lower NITs (National Institutes of Technology). JOSAA handles the initial seat allocation for IITs, NITs, IIEST, IIITs, and other GFTIs based on JEE Main ranks. If there are any vacant seats remaining after the JoSAA rounds, CSAB conducts special rounds to fill those, also based on JEE Main scores. 

With 3 Lacs CRL and 11k OBC-NCL rank, chances in JoSAA are low for top branches. But don't skip counseling. Try filling all NITs, IIITs, and even lower branches. Also check CSAB special rounds later. You might still get a seat if some colleges have vacant spots.

IIIT ALLAHABAD, IIITM GWALIOR, IIIT DELHI and so on .

Please share your crl rank .

And I don't think in jossa you will get .you can try in CSAB .