Terminal D took 5 years (appx) to build, so I think the 5-6 year estimate here is very realistic.
I'm guessing they will have to tear down the current Skylink elevated track in order to build the terminal? But maybe with the F terminal only being 24 gates, they are accounting for building inside / around that. I'm sure that would make the construction process go a little slower.
Terminal D has 27 gates, so 24 on the new terminal seems realistic... especially if it's all widebody.
I do think it's very possible that there's no customs / immigration at the new terminal and instead is just a people mover over to D. Then again, with AA's expansion into Central America lately out of DFW, I'm guessing they would like another CBP location at KDFW.