Nozzle hits prints off after about 450mm
There is gunk that can accumulate on print sheets. PLA brands that leave a white residue that require alcohol or acetone to dissolve. Or after long periods of use the PEI itself oxidizes and requires acetone to dissolve and the soap and water to remove what the acetone leaves behind. But you've said this is a relatively new printer.
I have hard water and have a water softener. If I rinse with 100% hot water, the softened tap, the soap is never fully rinsed and water doesn't sheet off as well as I want it to. So I have to mix in some cold water, which is plumbed to bypass the softener, and seems to remove the soap film and leads to better sheeting. If you have naturally soft water, you might try using less detergent. A drop or two on the paper towel is usually more than enough to lift oils.
As for Windex - I've tried it for general sheet cleaning, and I've never been as happy with the results as plain old soap and water. If you think your brand left residue, try wiping the sheet down with acetone. Pour a small puddle of acetone directly on to the sheet, start with 3 to 5 cm, then quickly wipe it around the entire sheet. Use appropriate gloves so finger oils don't wick through the paper towel; or use a good lintless cloth that acetone won't melt. Acetone may leave a streaky white film - I think it's the result of it removing PEI - and I wash with soap and water to remove that residue.
And - worth mentioning. I have had a poor experience with a roll of Prusa filament where I suspect the handler packaging it had recently applied hand lotion: the entire top surface of the spool was contaminated and refused to stick to the print sheet. Once the top layer was used up the filament began behaving.
Lastly: I need to keep in mind the notion 3D printing is art, not science. What works for one may not work for all.
I'm still not 100% sure why my sheet isn't beading off quite the same as the other clean side
At least you have a reference. We say Dawn or Fairy because the regular versions have few additives. There are 'Luxury' detergents with all sorts of additions that impede the cleaning action. You need a pure detergent, not a great deal of it, and clean HOT water. Rinse well.
Think about your working environment - what might be contaminating your printer and what cleans that?
We are assuming the contaminents are human fingerprints, filament residue and PEI oxidants.
But if you are using aerosols in the same room, or spraying paint, laying fibreglass, arc welding ... anything that creates dust, smoke, spray... You get the idea. The bed needs to be clean.
"As for Windex - I've tried it for general sheet cleaning, and I've never been as happy with the results as plain old soap and water. "
It is not useful for sheet cleaning. I would recommend that you reserve it for PETGs that over adhere. It is good as a release agent and to reduce over adherence.
3D Printer Review Blog...
The glue stick is to make it easier to REMOVE prints not keep them on. Right?
Yes and no. FDM printing is a maze of facts, factoids, and factors.
The gluestick as provided by Prusa is to help with Nylon or ABS adhesion on the smooth PEI print sheet. It can be used in place of the noxious ABS Juice(a slurry of ABS and acetone, near as I can tell).
However, when something is sticking more than your want, like some PETG brands, the gluestick becomes a buffer that reduces adhesion.
https://cdn.prusa3d.com/downloads/manual/prusa3d_manual_mk3_en_3_04.pdf (page 16, and others).