When you're wiping a headlamp lens, you're not wiping the plastic. The plastic is sealed off from reach by the hardcoat, which is extremely hard. Yes, it can be scratched, but it takes quite some effort. This stuff about "no matter how soft the cloth runs the risk of scratching the plastic" just isn't accurate unless we're talking about dry-wiping a severely dirty headlamp. Wash with soapy water and a soft-bristle scrub brush and/or reasonable cloth, and there is no concern.
I watched a truck driver destroy a brand new set of Peterbilt headlamps in 6 months. Frequent wiping with microfiber and soapy water.
Mine which just turned one year old look brand new. And I wash them with the industrial pressure washer almost daily. Wax every couple weeks.
(emphasis added to highlight that this isn't advice based on facts, it's just a guess/assumption you're making)
Its based on years of fleet management accross hundreds of headlamps. If you have access too a study that looks at wiping vs washing headlamps as a long term cleaning method feel free to share. But the statistics I have based on fleet maintenance say the pressure washer wins out, hands down.
Most of our tank truck guys use microfibre for the front glass/headlamps. Which we find to be very hard on them. Whereas the combo units wash their headlamps off their own pressure washers and theirs last almost indefinitely. I dont have data on brushes, as i dont know of anyone using a brush on a regular basis.
Power washers available to the general public probably won't harm hardcoat or a lens in basically sound condition, but that much water at those kinds of pressures can easily create other problems -- like water ingress into the lamp or its vent systems.
No more or less than you would be washing the rest of the vehicle with a pressure washer. Wash the engine bay and expect some condensation. OEM headlamps with intact venting systems will clear up in a day or two with use.
I think it should be obvious that you want to avoid jamming the tip of the washgun right against lenses and seams. No different than anything else. You wouldnt jam the tip into your toes either.
If you reaaaaaaaaaaaaally want to use a cloth as opposed to a pressure washer carry a spray bottle with soapy water and spray the headlamps off excessively before wiping (remember dishsoap removes any waxes you use). But if the headlamps are dirty odds are the rest of the vehicle could use a bath too, and you arent wiping your paint down with a spray bottle and a chunk of microfiber.