Wiring – Splice new outlet branch in attic

In turn here:

• For your case b), conduit is not required -- NM is routinely run through bored holes in structural members without conduit.
• For d), again, conduit is not required -- you can get away without guards if you run the cable along the side of the joist and at least 1.25" away from its edges. (320.23(B) and 300.4(D))
• As to your protective runs in c) and e) -- 3/4" Schedule 80 PVC conduit has 138mm^2 of usable room in it for a single cable or conductor, and a 6/2 W/G NM-B cable, according to your specs, takes up only 70mm^2, so you have ample space. This might be a tight pull, though, so you may wish to go with 1" instead.
• There are two other things you need to address with your conduit runs:
  1. The conduit you linked is Schedule 40, which isn't sturdy enough to protect wires from physical damage -- the code requires the thicker, sturdier Schedule 80 instead.
  2. Make sure to put expansion fittings in your PVC runs lest they do the snake on you mid-summer, or pull apart mid-winter! Easy rule of thumb for conduit that is operating within oh, 24degC of room temp -- put them in every 4m/12'. (You might be able to get away without it for isolated runs like yours provided the supports are set up so they don't provide excessive longitudinal restraint to the conduit.)

Replace the elbow-piece -- it's really a type of conduit body called a LB (L for elbow, B for out the back) -- with a box and make the connections to the new wiring there. You may have to pull a new stub-length of THHN to the existing receptacles to get enough slack in the box on the existing incoming wires to make the junction.

As to the new run -- I'd use two lengths of conduit for the exposed run: one from the tap box to a box mid-span that serves as a tap point (I'd use a T body, but there's not enough space for it unless the conduit run is offset sideways from the switch location anyway) and the other from the upper box (it can be a handy-box as the box fill's minimal) to the attic, then use a nipple to stub off from the back of said handy-box to the box with the switch in it (which'll need to be capable of taking a conduit). A one-gang box should be more than adequate for the switch box, by the way. The wire would be THHN to match the existing circuit gauge, in standard colors (black/white) of course, with only hot (black) wires run to the switch location from the tap box behind it.