re: ET itself into orbit?
It had been argued even long before the Shuttle program, with Skylab or rather the AAP.
The problems are the mutiple openings required and emptying the tank in LEO (and weightlessness).
OTOH, the ET is simple, big , not even painted...and not that expensive.
If openings are added, weight is also added and would it be simetrical?
Costs rise, then!
It's not because it's huge, simple and not reusable that it must become complicated and left in space for further uses.
Raoul