If you look it up, ethanol (assuming you are referring to ethanol when you say alcohol) molecules are about 4.4 Angstroms in diameter. Isopropyl alcohol is about 16 Angstroms in diameter. 16 > 4.4.
Cleaning effect depends on what you are trying to clean. It depends on both solvent and solute. Like dissolves like. No one solvent is appropriate for all cleaning uses.
If “impure substances” are causing you problems, you’re buying the wrong grades of solvents.
Nope… Wearing gloves is OK, that’s what they’re for! Even if your palm are sweaty, knees weak and arms are heavy 
For handling stuff - again - gloves. But since now we’re talking about dry stuff, then there are cotton gloves, the delicate kind. Here they’re called “jeweller’s gloves”. Breathable and still won’t leave oils on surfaces.
Wearing gloves is even advisable depending on what people are doing and what chemicals are involved.
Anyone here remember DMSO? It’s kind of an extreme example but it’s a solvent with both polar (water-like) as well as non-polar (oil-like) properties. What it’s mostly known for in public circles is drug delivery. It helps all sorts of chemicals right across the skin barrier and into the body.
Alcohols can do similar things. One thing it does is reduce the barrier effect of cell membranes which indirectly helps stuff move into and out of cells. As people use various exotic chemicals and materials with printing, it’s wise to minimize exposure to chemicals that can help stuff into the body.
Appropriate gloves can minimize the risks.
So far I have only had to wash my plate once in 2 years of printing, simply by only holding the front edges of the plate while using it. But I can tell washing introduces even more chances of hands touching the plate.
Here’s the how I did that one wash a few months ago (yes, it’s a lot of trouble to go through, the reason I want to avoid having to do any washing at all).
Cleaned a big area of my counter top, then wiped it with 91% IPA, then wiped it dry with clean paper towel. This area was further divided into two areas, each covered with clean paper towels. Each of the two areas has their own purpose in later steps.
Washed the plate with liquid soap with paper towel. I dindn’t use the sponge because it was used for cleaning dishes and would not be free of grease.
held the front edge of the washed plate and rinsed it.
Put the rinsed plate on one area of the previously prepared counter top. Dried the plate with several sheets of paper towel, making sure hands were well above the plate. Then moved the dried plate to another area of the couter top that was covered with clean paper towels. Let it sit some more time until both sides of the plate were completely dry before using.
Well, I definitely can’t explain that. I’ve had a very different experience. Are you using a smooth PEI plate with no adhesive, or some 3rd party thing?
With the textured plate that I usually use, I can rarely get by using the same area of the plate 4-5 times without washing it with dish soap, before I start to have adhesion issues.
My understanding is/was(?) that filament leaves trace amounts of oils (or something) that build up and cause problems.
The very first black textured PEI plate that came with the printer. I believe this plate has been discontinued because many seem to have bad experience with it. But mine has been working well. This is a two-sided plate and I am still on the first side. It probably will outlast the printer.
@MZip: You are wrong - long ago, in the days of mainframe computers with large platter disks, only isopropanol (recommended by IBM) was used to clean individual disk heads. Ethanol, due to its larger molecule and slower evaporation, left traces on the ceramic surface of the head, which led to errors in reading and writing to the magnetic track of the disk. But you have your own opinion on the issue and I will not refute it.