Welcome to FT oneworldboeing.

But this top about where and what to do.. has been discussed many many times before. Please do a search by clicking on the upper right hand corner of your screen and you'll find all your answers you need.

There is no diff to book through airlines direct or travel agents though the latter might offer you a small discount.

Yes it's even cheaper than BKK if you start from CMB. But you'd have to get a ticket to CMB to begin with.

For your routing.. you can only stop over twice not including origin.