According to the manual, when putting the slide back on, you do not have to grip and hold the barrel to be able to slide it back onto the frame.  However, the youtube videos all seem to demonstrate putting it back on while holding the barrel.

For me, I have to hold the barrel for it to slide back on...but it sure would be easier not to have to hold it.

How about for you?

I too have struggled with the reassembly, especially with my old weak hands.  The first time I tried it took me at least 15 minutes to get it on and I was not sure what l did to make it go on.  After spending some time practicing l found that if l get to the point where the slide stops, l change my grip to an overhand grip and then proceed to rack the slide.  This will push the barrel forward slightly and allow the firing pin block to rock down and slide over top of the frame rails.

I got my R51 back together. After I finally figured out that the barrel needed to be pulled forward.
It did not say that in Remington's video. 

Go to wallyworld and but one of those little spring clamps with the plastic covers on the tips, and when you first rack the slide, clamp it to the end of the barrel.

Then drop the magazine and remove the take down pin and the slide will come right off no problem.

use the clamp for re-assembly and you will have no problems.