Buses don't use the East Bank Station platforms, but they do operate on the tracks going through the station.I'd probably prefer your concept of turning 5th over to transit, but the current plans are not completely discordant, and I think that's why there hasn't been a rush to propose an alternative. The Green Line represents an existing condition, and the Rush, Gold, and Riverview routes kinda do agree with each other. The idea of split one-way lines is nothing new and works in cities all over. No aBRT nor the Red Rock Corridor is currently in serious planning, and when other projects come up, they can adapt to the existing set-up easily by following the N-S and E-W trails already blazed by the existing projects.One thing that increasingly concerns me (and now I'm going to be the broken record) is the lack of a coherent transit plan for Downtown St. Paul.
There's a lot of synergy already. All three take the existing trunk routes on 5th and 6th as a starting point. Gold and Riverview will stop at or near the existing Central Station and the Union Depot. Both are also envisioned to stop Rice Park and at Kellogg (although, as I mentioned above, I think the Rice Park stop should be eliminated). I haven't seen it made official anywhere, but it's obvious that these stops could and should serve both modes on one platform, just as buses use the East Bank Station at the U. Even though the Gold Line is still in design, Riverview is so far out that it might be tricky to build the(se) Gold Line station(s) to LRT specifications, but I'm sure they could be designed in a way to allow future expansion.
The Rush/Purple Line comes into downtown at a slightly different angle, but it will share the Gold Line stations at Robert St, the Green Line Robert Street Station (I think this stop will eventually be renamed), and will also serve the Union Depot. So the only spot where all four services meet is the Union Depot, but that's probably fine, and will certainly make Commissioner Ortega happy.
I'm not too familiar with how transit routes are configured in St. Paul, but is there something like Marq2 in Minneapolis, or is something like that being proposed for Gold/Rush/Riverview?