If the B3 friction disks are worn or in some cases have come apart, the K1 clutch pack will rub against the B3 piston causing a jerky pulsing motion in reverse. That coupled with the fact that it takes a long time to engage tells me that you probably have some serious problems.
I had similar problems with my 91 420sel. When I opened the filter there were large pieces of the B3 friction disks in the filter. It was interesting that there was no indication that these large particles were there until I cut open the filter and examined it closely. The filter did a good job of catching these particles. There was nothing in the pan what so ever, it looked clean. Needless to say I was horrified when I got the filter open.
However your car is much newer than mine and I cant say if my experiences apply in your case.
I got too many cars!! Insurance eats me alive. Dave
78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k