I'm not currently aware of a door for this unit, you will need to use a Curved Rod and Curtain.
Hello JB. I'm sorry, I don't have access to Home Depot's stocking levels. You will need to contact the local store to get that information.
It is a relative question...it would be cozy! But probably. If this is a remodel, be aware that this is a One Piece unit and you will need room to move it through the jobsite. If you are New Construction it should not be a problem. You may want to look at the 6042BWC. This is a Tub Only, but is setup for 2 person and you can tile the walls.
At the center point of the curve, you are at 42" deep from front to back
They are actually grooved, however, it is one continues, surface and is very shallow. After you clean the unit, you may choose to WAX and buff the walls, this will help keep them cleaner longer (like your car surface). A regular cleaning routine will keep the work to a minimum. I've installed a tiled version in my Mothers house and we simply spray it down with a cleaning solution and do a simple wipe down and it stays very clean.
From the Top of the Drain to the Bottom of the overflow is 12.25"
Your center point for the drain would be 21 1/8" from the Back and 8 3/4" from the side.
The shelves are 2 7/8" at their widest point.
If you go to either the HomeDepot website or the aquaticbath.com website, you can pull up the specification drawing and it will give you a Top Down view to get a better idea of what it looks like, but to answer you question, the Wall Shelves are only along the Backwall portion. The Tub also has a decking all the wall around to give you additional storage space.
The dimension from the Top of Drain to Bottom of overflow, is approximately 12.25".