Can I use heating mat to promote germination?

I didn't see any need to deviate from the directions. Have ordered several of these tree's and have had a 80%-100% germination rate per container. They take time though.

Can they be grown indoors as bonsai?

Your seed is started indoors to take advantage of your homes consistent warm temperatures, that are augmented by putting your assembled (and pre cold stratified) kit in the sun for even more heat. The goal is to germinate your seed, transplant it and then transition it to out of doors. Giant sequoia make great bonsai but as they are not a tropical plant they can not be grown within your home long term.

Which month is the best to put the seeds in the fridge? I live in a climate in which it doesn't dip below 40degrees F.

You can plant the seeds any time being that you are simulating cold stratification by placing them in the fridge but it is highly recommended by The Jonsteen Company to plant in the natural cycle of the season which would be spring.