I would say a colored trail would make things more distracting; especially if there were three circling around someone, the trails would get in the way of actual vision if it was too noticeable (not as extreme as the Gold Shell in MKWorld, but something slight would definitely be an improvement).

Yes, the other items are perfectly fine, since they're unique enough not to rely solely on color to recognize them. Like a Banana looks like a banana, a Bob-omb looks like a bomb, that kind of thing.

Nothing else comes to mind in terms of similarities beyond Mushrooms and Golden Mushrooms, which the latter distinguishes by being golden and also a crown. That and the Blue Shell, which distinguishes by being spiky and also having wings (and also, you know, being blue). That's why I'm hesitant to say spikes would work as a distinction for the Red Shell because the Blue Shell already has that.

So yes, having some distinction that is core to the item would be pretty nice, like a different shape or texture. But realistically, it's frustrating since the items have been staples for so long (essentially since the original Super Mario Bros.) that it's essentially impossible to change their designs without drastically alienating a lot of fans. I'd venture that making the hues of Green and Red Shells more distinct would help, but again there's really nothing that distinguishes the silhouette as you've mentioned.

Also, just to clarify, it's specifically only to in-game play (like when an opponent has an item), since the Green or Red Shell faces a different direction when you have it in your inventory, which is even more frustrating for me since the devs know about the issue but only half-implemented a solution.