You should choose BTech only because it is recognised more compared to dual degree courses there for you will be given more opportunities and treated different from the dual degree course therefore go for BTech only because you will miss some opportunities that on the BTech students that also in case if you are not interested in chemical you can do your masters in other courses.

It depends upon you if you would like to continue M. Tech after B. Tech in the same Institute then you can go for B. Tech + M. Tech.

If you want to change your Institute after B. Tech then you can go for only B. Tech after which you would have to give entrance exam to get admission in other Institute for M. Tech. 

If you want to look out for other options after B. Tech then you can go for it alone otherwise for the dual degree (B. Tech+M.tech)

It totally depends upon you what would you like to do.

If you still have any queries feel free to ask in the comment section down below.