I think you hit the nail on the head many times with your post.
I don't think one would find and need 7 th order AA filters in the usual scopes, at least not the classical butterworth type (there are usually a few parasitic limitations involved anyway). Usually the amplitude to higher frequency goes down and some low level aliasing is accepted. So the AA filter is not suppressing down to the full ADC resolution. I would not expect much visible difference if only good for 5 bits.
The probes supplied with the entry level scopes contribute a lot to the BW limitation, that is in real life the trouble is more to get enough BW than that aliasing has noticeable effect to signal shape. Especially that harmonics have smaller and smaller amplitude both by nature and due to BW limitation. There is one place though where aliasing do show up quite remarkable and that's the FFT output. This is due to that some of the scopes have excellent dynamic range (approx. 100dB) after the FFT.
Normally it's more an issue to get the probing to such level that the output is not a degraded sine with fast signals. While it's very easy to provoke scopes via hanging a purpose made circuit to the BNC input that outputs a low fundamental but fast rise time signal probably nothing could be further away from the typical real life use than this.
There is though one user error that can get quite much overshoot/ringing: Impedance misalignments and reflections. These scopes typically have only high impedance input.
Switching the speed between 1 or 2 channels per ADC (or the other way around) would need switching the AA filter, but the steps further down when the sampling rate is reduced because of limited memory / low horizontal speed usually use digital averaging, so that no extra filter is needed. With some of the cheap scopes the sampling rate is no longer that limiting so no need to use 50% or even 80% of the Nyquist limit. Using a relatively large fraction of the Nyquist limit is more like a thing of the mid class scopes at some 500 MHz BW. The limited ADC speed was a big thing in the early days (1990s).
Although I've brought up the idea in one of these threads, I pretty heavily suspect that few scopes (especially in entry level) implement filter switching when ADC has to be divided among multiple channels and as a consequence sampling rate is reduced per channel.
There are many hard to answer but interesting questions that you've brought up. Most VGAs have separate output for triggering where a lower rate ADC could be used. I don't remember any comparison or even advantage/disadvantage summary of this or the main channel FPGA based digital triggering.
For the reduced sample rate due to acquisition memory limitation two (multi) stage properly filtered decimation could indeed provide excellent result but does it fit to FPGAs of entry level scopes?