why this *2 factor?

This is basically repeating the signXecUtxosByAddress function

Are we not able to pass inputUtxos and tokenUtxos in one array to this function?

remainder should be defined before the above for loop (as just let remainder), and then defined before the if block that breaks the for loop if enough utxos have been collected.

This way, we do not need to repeat this definition. The if check in the above for loop should just be if (remainder.gte(0)) {break}

I haven't tested this but I suppose it would work. remainder is a BigNumber here, so let's stick with BigNumber methods.

https://mikemcl.github.io/bignumber.js/

i.e. remainder.toNumber()