Half the Bangkok flight from Bombay is full of people from Ahmedabad doing the exact same thing what you are suggesting. 985 is amd Bom mct flight and lands at international terminal. Your bags will be checked in all the way to Bangkok. You could also take the later flight 191 and reduce your ground halt in Bombay airport which also lands at the international terminal.

On the way back the cabin crew is the same on the Bangkok Ahmedabad flight and it is the same aircraft also. Though they will make you get down and board again however you should not be concerned about missing it at all. Again your bags will be checked in all the way to Ahmedabad and the fligh will be landing at international terminal in Bombay only.

Your only alternate option could be a spice jet flight which I heard were launching a direct Bangkok Ahmedabad flight. However this was a very bad rumour I heard so don't count on it.