It can also be confusing if you check the volumes after mixing, since the water-alcohol interaction decreases the molecular spacing and the total volume. V_(water + alcohol) < V_water + V_alcohol.
Normally, yes. Today, I’m not even able to find isopropyl at many stores. I’m actually thinking about ordering flux cleaner for my soldering, for the first time in my life.
You can get methanol pure and cheap from auto stores (in regions that get cold, anyway). Just try not to get it on your skin or breath the fumes too much. It's quite a bit more toxic. Definitely don't use it to make hand sanitizer.
Mixing on a scale can be pretty confusing, though, because the percentages are (I think) by volume, and water and alcohol have different densities.