... I'm particularly confused why some amp tweakers are charging $1100 for this. Like if I just wanted the tone for one channel, it really looks as simple as changing two gain stage's cathode bypass values and maybe a couple other small tweaks. But nothing that warrants $1100 lol.
You're able to tinker your own amp, so you're not the target audience. Cost factor could be a number of things, including the ability of customers to pay (and the cost of living where the modder is located).
I haven't owned a Gibson Les Paul for almost 20 years because of Gibson's price structure/philosophy. Which also amounts to "I can't afford to buy one." But this year I'm able to afford one and really wanted a Les Paul, so I bought a custom shop model. I could have bought a different model and some outstanding pickups and had the same sound, possibly same playability. And I may still do that in the future.
Coming back from the detour, we don't know the schematic you have is "the mod". Or even if there is only one "Arredondo Mod" (though some folks on others forums dedicated to only Marshalls might know). It might be a case like Dumble where the high price isn't justified by the intrinsic value of the amp/mod itself, but the fact that the modder works directly with an artist, listens to their vague descriptions of what they want, and custom-engineer something which delivers it. And usually those situations don't mean engineering the solution once, but going through several iterations as the customer tests out the amp and asks for further changes. That's a lot of time spent, and bench time is the biggest % of any repair/mod bill (for guitars/amps as well as cars).
My new Les Paul is pretty sweet, but part of the choice to go custom shop is the high resale value if I choose to do that compared to other options. The name/model is held in high esteem so I don't have to worry about depreciation as much. I don't know which modders you were talking about, but some are famous enough to add resale value to an amp they've modded. And some changes may be valued enough by the present market (legitimately or not) that an amp tweaker might have customers even though the work is priced very high.
Going back to the Les Paul comparison, I saw a guitar "conversion" recently that made me laugh. Someone took probably a Les Paul Classic, refinished the top, installed different pickups and "crap-ified" (my opinion) the hardware/finish to create an aged effect. They were charging $3k more for their aged guitar than the price of a used LP Classic. Someone might value the conversion that highly; I doubt it will fare well in the resale market 10-15 years from now.
Most of the above is to say the price people charge for mods is immaterial to you if you're just looking to understand how/why something is changed or to make the change to your own amp. The economics only matter for you if you're paying someone to do the work, or looking to make a living doing it yourself.