Costly? How data card is costly?
Also wired broadband is stable in my opinion, I have an home office and also do stock trading, so stability is a requirement for me, good latency too
Merging is not easy. The lowest cost you will find for a Dual WAN router is 7K. As for using BSNL as a backup, see how BSNL performs in your area. I hope you understand that support is almost non existent in BSNL's case. Personally, I would check if there is any 3rd option available.