Motorcycle helmets are streets ahead in safety design and also on price - due to volume and competition. My daily commute inside a lid with built-in Bluetooth communication to radio, pillion, phone, satnav (noise cancelling headphone) sees me paying much less than for an "aviation" specialist lid (i.e. £450 current price).
Also, there are motorcycle lids specifically designed to handle high heat / slower speed airflow combinations. One open-face lid by BMW has a steel mesh panel to aid cooling (
see here). I have this and use it in summer and can vouch for its performance.
The suggestion to change the helmet every 2 years is due to compression of the comfort foam lining material through ordinary use of the helmet. The helmet protects you by slowing the rate of deceleration of your brain as you bounce-off something solid. Hence the outer shell must be strong, and the inner must deform during impact so that your head and the outer shell decelerate at different rates. If the foam comfort lining is loose then the efficiency of the helmet is reduced (by the way, the difference between deceleration rates in a collision 'bald v hair' is measurable).
If you want to see what a small gap will do in terms of changing the impact consequences try firing a 12g shotgun correctly and then a second time with a weak contact between stock and shoulder. The bruising from the second shot should recede in 3-5 days.
If you have an impact wearing the helmet then the deformation of the inner (which saved your brain) might not be visible, but it has done its job and will not be as efficient next time. If you drop the helmet you might weaken the outer shell and it might not perform in a later accident.
Shells are made from different material which (no surprise here) react different to thinners in paint. Polycarbonate shells are weakened by cellulose paint / thinner, for example. You can paint them but specialist plastic primers should be used. Polycarbonate is weakened by petrol, and there have in the past been instances in UK where the helmet broke in an accident at the point of attachment of the chin strap, due to repeated exposure of the shell to petrol through the hands of the person after filling-up. Fiberglass shells are not the same, and Kevlar or carbon fibre shells are different again. Be careful to check that the shell type and the paint are compatible. Anyone offering paint service for motorcycle helmets should be competent to advise on paint / shell combinations.