Regarding the gas tube, so if I bought a complete 16" carbine, and wanted a 16" mid-length gas system (for whatever reason), I'd have to buy a new barrel as well as the gas system?
The "gas system" you're referring to is simply the tube, about $15. The expensive part of this swap will be the barrel. So long as both barrels have the same OD at the gas port, everything else is interchangeable.
Changing a barrel is not difficult. You just need a small hammer, a couple of punches (1/16" and 3/32"), a vice, an action block (or some soft pieces of wood, and an armorer's wrench (about $25, unless you're going to use it a ton; That would warrant a more expensive one).
1) Remove handguards
2) remove flash hider/brake/compensator
3) drive gas tube pin out of front sight base or gas block
4) drive out pins holding front sight base/gas block to barrel (unless you have a set screw fastened low profile gas block) and remove FSB/GB
5) Secure recevier in vice and loosen barrel nut. Unscrew and remove barrel
The only part that can be trying is getting the barrel nut aligned with the gas tube hole in the receiver. The amount of torque required to go from minimum and "oh, I just passed the last notch" to lining up the next notch can be
A LOT. This step becomes even more tedious if installing a free float quad rail or drop in quad rail handguards that align on the barrel nut, as being off by the tiniest amount will be noticeable in the rail alignment.