I made a quick picture for reference (please don't judge, I'm not a graphic designer..). You might be familiar with what follows, but in my experience not all of this is common knowledge.
All engineers know that the center of a guitar speaker is brighter than the sides. It's true, but not nearly the whole story. Even though the speaker is round, it doesn't emit the same sound in every direction 360 degrees. See the two little glue tabs on top of the dust cap? That's where the positive and negative speaker wires are attached, the terminals are on the other side. Those two glue tabs work exactly like moongel or duct tape on a drum head, they dampen overtones. Positions 2 and 4 don't sound the same, rather 2 is noticeably brighter than 4, and 3 somewhere in between. With brighter tone the difference is of course more evident, but it's audible with any amp, any speaker.
In 1x12 combo amps, the glue tabs are most always on top. Because the speaker wires go up, it's the shortest path to the amp and the only sensible way to put in the speaker. However, in 2 or 4 element cabinets there's variation. In horizontal two element cabinets the glue tabs are usually facing each other, again because it's the shortest way for conventional wiring. With four speaker cabinets, the left and right speakers are usually facing each other. Notable exception would be Mesa/Boogie, who wire their 4x12 cabinets with the speaker terminals of all four speakers facing towards the center of the cabinet. I've also seen 2x12 / 2x10 cabinets with the speaker terminals pointing up. With vintage amps it's a gamble, and of course if someone changed the speakers to their cabinet, they could be installed in any direction. Many people tend to look at the brand logo and rather have that straight...

The only way to know is to look with a flashlight. If two players have a balanced sound when listening in front of the amp, and using the same mic within the same distance, in the same position you get a wildly different capture, look at the glue tabs and see which way they are, it might be that your miking one player's speaker from the opposite side of the glue tabs / speaker terminals.
Here's my subjective take on the seven positions. I only use positions 5-7 if I want a dull and distant tone (guitar is not the featured element or main accompany in the music), or need to compensate an insanely bright sound that the player is unwilling to change. Even though there's more low frequencies (mostly low mids) in relation to high frequencies, miking the paper yields also the smallest tone. Positions 2-4 are the usual go-to's. For an SM57 (which I regard an excellent guitar cab mic), 4 or 3 are good, 2 is usually too bright, unless the player has a dull tone that needs to be compensated. Position 2 or 3 would usually be good for an M160, M88TG or any other beefy mic which might benefit from some bite. For most any common dynamic mic, dead center is way too hairy. However, that's where I stick a Royer R121 ribbon when recording in studio, and it's an awesome sound. Air really is the most natural sounding compressor, so pulling back the mic not only reduces proximity effect but tames the sound some, as does angling the mic. I don't remember the last time I angled the mic though, knowing how the speaker behaves has given me enough variations to work with.
Once I figured all this out, I stopped listening to sound demos of mics and guitar cabs. I haven't seen one where the presenter would make it clear from which side of the dust cap they're miking the speaker. Also, in many demos where a bunch of mics are recorded at the same time, they're positioned around the dust cap, I guess I don't have to underline what's the problem with that.
About the acrylic baffles, yeah... As you noticed, positioned too close they tend to create catastrophic reflections and in my opinion should be the last desperate resort to fight volume. I love my '59 4x10 Bassman and Bogner 4x12's, but bringing a right size amp for the gig really is the way.

What players fighting with volume should consider, is putting in a speaker with 3dB lower sensitivity. This way they'd effectively cut the wattage of their amp to half. It will not make a 50 watt, or even a 5 watt tube amp a bedroom amp, but at modest rock band volume levels, dropping the overall volume by that much would be huge.