hey
Yes signalling bus for global relay flip just need diodes.
But there is some under laying consequence for the line/mic flip, basically the line gain "have to" be fixed (studio line level unity)
to "truly" switch to line mixer/multitrack playback, don't now if your line input share the same gain sw as mic for adjust, but maybe to make it efficient and fast when flipping all, a fixed gain network have to be switched too when your line become a tape return.
Insert/DO, my first thinking is two different item XLR (sym) DO and TRS unbal send return.
Second idea is to combine DO/send sym, with cut or pass trough switch, but then it will be better to have return sym as well...
The better is both sym but then you need 3 trafo and 3 XLR at back per chanel, which could be to much space for small side car
(there is also Dsub option to gain space)
All option open, so it really depend at your side what is possible and not, with design constrain.
Regarding routing, I don't see the point to have DO post fader, because you then miss all the interest to have multitrack out AND stereo mixing and monitoring. But that's me and my workflow, let's ear what other says
Fader so soon in the chain is problematic if you want extended routing and option in "modules" position (EQ/insert)
I'm not used to tube design with load etc, so I understand they might be design limitation
But I think I get it, you need DO post makeup anyway and having fader post DO let the fader without buffer.
If the cost is just insert at master post fader (and not pre) I'll say it worth it...
VE summing have another advantage is that if you want a side car to your side car, you can link bus if you anticipate option at bus board

regarding aux i'm not sure to undestand ? you mean one send only but switchable between two aux bus ?
I'll say if you have the space at FP, and layout room at modules pcb and buss board to have have 3 aux bus (meaning you can have 1 mono and one stereo or 3 mono) it will be just fine !
Best
Zam