From your description, it sounds as though you've used these socks successfully many times without the rash and pustules. So, the riddle is why this time.
Aside from the ingrained pattern, the irritated redness looks a bit like a combination of heat rash and friction. You mentioned working (skiing) harder and warm temps, which might fit with a heat related rash. The pustules do look like inflamed/blocked hair follicles/sweat glands. If you look closely, there appear to be a few other slightly blocked follicles within the rash. Possibly this condition is all tied to warmer weather and working harder, leading to more acrid perspiration and tissue swelling. During high levels of cardiovascular activity - especially when warm - I notice some tissue swelling as blood perfusion increases. My ski boots and cycling shoes become tighter in these situations. They're already tight to start with, so I usually need to make minor adjustments or pop the instep buckles for the ride up.
Are you using the compression Eurosock or standard? If compression, it wouldn't be uncommon for a mid-weight weave to leave an impression on your skin. Even if not using the compression sock, if it is stretched tight around your calves/shin the same can occur. To "press" the pattern into your skin from the boot, the pressure would have to be consistent, not giving your skin a chance to rebound. Not a sign of being in the back seat all day if the pattern is on your shin. So either the sock is "compression tight" or your boot is in that area. Sounds like you ski frequently, at least 1-2 days every week, so by the time the crosshatch starts to subside you re-emboss it, making it stay for the season. Good for you! :)
The rash could also be a reaction of more active perspiration than normal in conjunction with residual laundry detergent trapped in the fibers. Some cyclists are sensitive to this with tight fitting lycra and switching laundry products (or not fully rinsing after washing).
Hope you find the cause, but don't stop skiing!