Some of the M3501s have Mitsumi switches that are supposed to be disappointing but unfortunately I don't know if they're mitsumi alps copies or whether they look different.
What's the difference between the Dell AT101W ALPS switches and the ones on the AEKII?
Assuming it's using Alps switches then it should have an unbraced tactile/click leaf instead of a braced one so it clicks as well as having rubber dampers on the slider. It may also have a different spring but I don't know if anyone has catalogued the Spring differences in Alps switches.
Oh, and the keycaps of course. I don't know if the AEKII uses the heavy ones of the SGI AT101 but they'll almost certainly be different to the ones that the Dell uses which will have an effect too.
Is there a different model of the AEKII (or any other mac keyboard) I should be looking for to get as close to an AT101W experience as possible?
As I mentioned above the AEKII comes with both Alps and Mitsumi keyboards and the AEKI also uses Alps. I believe the Macally MK-96 also uses alps.
Is there a way to replace the ALPS switches on the AEKII? Where would I source the switches?
Plate mounted alps switches can be opened without removing them first and the guts of the various types of real alps are interchangable so if you're definitely got the genuine Alps switches (and maybe the Mitsumis as well if they're copies) then you can swap their guts for those of a Dell AT101. If you don't mind the appearance then swapping the keycaps is likely to help as well. Some types of simplified Alps will suffice as well (the Strong Man Type 1s definitely work). Leave the white bit alone, that doesn't come out.
There's a guide to opening alps
here.