I love the mock up of building a lightsaber. I think it would be a brilliant addition to the start of RotJ. The idea of the cave is also great, but thus far I don't think anyone has made an edit where they include newly filmed scenes of any sort so as much as I like the idea, I think editing that can be done by a one man team at a computer just seems more realistic (for the moment).
That said I think the idea suggested above of having Vader making contact with the Death Star from his ship, then having him attempt to contact Luke would work because it could save the Death Star reveal. But instead of having Vader's contact be followed by the deleted scene of Luke finishing his lightsaber I think it should be Luke in Yoda's hut. A shot from ESB of a closeup of Luke's face could be used and slowed with Vader's voice heard over the top. Then Yoda's voice could be heard talking about a jedi's weapon being a lightsaber (the biggest problem I can think of with this is I don't think Yoda says lightsaber in the films but perhaps a sound clip can be found from elsewhere). Then it could cut to a wide shot from ESB where Yoda is moving around in his hut and Luke is tasting some food. The reason for this is Yoda is facing the camera and looks like he is picking something up. He could tell Luke it is his old Lightsaber. The following shot in ESB is yoda turning slowly back towards Luke so I would suggest a line is constructed of Yoda then talking about the crystal being the heart of the lightsaber, and his crystal can then be shown floating towards Luke, with Yoda watching it. A shot from another film could then be used with just a closeup of a hand, with the crystal being added as if it is landing in Luke's hand. Yoda could then talk about building a lightsaber being all he has left to teach Luke.
I would then suggest cutting to the first shot of the Jabba plot, starting with Boussh (because R2 is meant to be on Dagobah with Luke).
Then at a suitable point cut back to Luke building his lightsaber, as shown in the mock up video. I think by darkening Luke's trousers (they would only be in half the shots) it can be made to look more like his RotJ clothing, and by lightening the ESB shots it gives the impression it is misty and early in the day (so as to mirror the darker shots when Yoda dies later on). Then it would play out roughly as in the video, but have Luke moving the pieces around to place the crystal inside what seems to be a mostly constructed lightsaber. Then he would hold it but not ignite it. a shot of Yoda could be edited to make him smile, and there are shots of Luke from ESB where his hand is just out of shot so the completed hilt could be added just creeping into view, and then the same with a standing shot. I made some quick stills to show what I mean.
Then it could cut back to either the Death Star reveal, Jabba, both, whatever. Then Luke igniting his lightsaber from the deleted scenes could be used, followed by his entering Yoda's hut tofind Yoda dying.
After that scene I would try an idea which I think originated in Spence's edit where there was a cut from Leia being in trouble in Jabba's palace to Luke turning whilst standing by his X-Wing on Dagobah, then cutting back to Leia - as if to suggest they are communicating through the force in the same way as the end of ESB. However, instead of the quick cuts I think Yoda's voice should be heard in the same way as Ben's voice was in ANH shortly after he died, with Yoda telling Luke to hurry as his friends need his help.
Sorry if that was a bit long winded but I think even though the symmetry isn't quite as nice as some of the suggestions such as having Vader appear in the cave, and Luke refuse to engage him, I just think these ideas are more possible because it is mostly just cutting and masking rather than filming new footage.