Modoc, it doesn't look like anyone has completely addressed your questions, so I'll take a crack at it.
The Marlin Jam is a timing issue, the factory parlance for it is "Letting In Two." The cure is outlined very well in the "Marauder13" sticky, and goes into lots more detail than I will. I did the fix on both of my 1894's in five minutes each with a file, a soft Dremel tip will also work. The idea is just to round off a little of the cam on the lever, not remove a bunch of metal. It is only a problem on the 1894's, no other guns seem to have the problem. It's due more to the short action feeding cycle, and doesn't cause problems in the longer actions.
The problem with jamming in the 35's Leverevolution ammo is due to the size of the mag tube. It's the same diameter as the tubes on the 30-30's and 375's, to accomodate the wider rim. Being rimless, the 35 doesn't need as much room, and can get cocked a bit sideways in the mag tube. This can trap the gummy tip of the bullet between the base of the round ahead of it and the tube wall. A thinner diamter tube would fix it, or maybe a sleeve inside the tube, but the new follower does help in some guns. The way you shove the LE rounds into the tube is also a factor, if you can keep them from canting sideways they feed just fine.
Cowboy shooters run into the problem because they shoot a lot. What happens (I think) is that the edge on the cam wears a groove on the underside of the carrier, and lets it dip a little in the middle of the feed cycle. That lets a second round slide into the action, tying it up. The fix is to pull out the screw that holds the lever in place, and clear the rounds, then reattach it and reload. I've heard it said that once you have a gun with problems, you need a new carrier, but I just smoothed out the groove, and they work fine. Not every gun develops this problem.
Hope this helps. If you have any other questions, I'll try to help.
Papajohn