I think you only have to put the stem with crown out, screw the tube out - measure it, look on eBay etc., order and put it back together. Or buy a used watch on m2m and you have all the parts you need...A Modder also takes his money. The parts you didn't need can be selled again. This is what I would do.
So you're telling this guy to remove the case back, depress the stem release button, remove the crown and stem, release the movement from the case, remove the tube from the case, using a digital scale measure the tubes lower diameter and upper diameter threads and the thread pitch, find a tube that has the corresponding measurements on Ebay , order it, reinsert the tube, movement, crown and stem and case back, without damaging anything - or else make it all even more complicated and order the exact same watch from the exact same factory just to remove the tube from the watch and sell it on M2M as a watch that needs a new tube ?
Unless I'm mistaken, option 1 is not viable because OP didn't even know that a tube is a fairly simple thing to replace, therefore the likelihood of him completing all of the steps in option 1 are highly unlikely.
Option 2, well, he could just sell his watch on M2M as a watch that needs a new tube, and with the proceeds buy a new watch - much quicker than what you've suggested, and gives pretty much the same result.