Greetings. On the question of the lens yellowing; The lens is made of Polycarbonate plastic. Tough stuff. It can take LOTS of abuse (see previous comments about the basebaLL bat).
But PC is softer than other plastics (like Acrylic). In order to meet the DOT spec, the headlight lens must be "Hard Coated". This coating protects the surface of the plastic against abrasion, UV hazing, etc. With proper care, the lens will remain clear and bright for years. So, with hard coating, you get super tough PC performance and long life as a result.
"Proper care": I'm sure that you have seen plastic lenses on auto headlights for years. They should have been hard coated too. Some are not only yellow, but they are so cloudy that they seem more like a plug in nightlight than an effective headlight. The discoloration and cloudiness can be due to a lot of things. But common causes include severe abrasion from road grit or severe mechanical cleaning (with abrasives) or CHEMICAL cleaning. If the hard coating was not applied properly or if the type of hard coating was ineffective, the lens will degrade fairly quickly.
Good hard coating applied properly (quality) will prevent issues due to road abrasion for a long time. But, you can ruin a good lens in minutes with the wrong cleaners.
Again, to avoid a direct "commercial", I would suggest that you do an internet search on compatible cleaners for Polycarbonte plastic. Some lighting companies and plastics companies publish this information on their websites.
A lot of the other plastic lenses on our bikes are Acrylic plastic. This type of plastic is not as sensitve to chemical attack. BUT, acrylic is harder and can be quite brittle. It's a trade off.
I am over simplifying - so before a reader who happens to be an Engineer goes after my throat on this, I admit that I am generalizing and that there are some types of VERY tough Acrylic plastics that come close to PC performance in the area of impact toughness.
But, in general, the plastic used for light lenses, especially when low production cost is an issue, tends to be a type of acrylic that shatters fairly easily on impact... Again, in general, the average lens is no where near as resiliant to direct impact as a lens made of PC.. but they are not as sensitive to chemical problems either.
So, be nice to your Polycarbonate headlight lens... the other lenses on our bikes are generally not an issue.
Most bike washing products are just fine. But if you accidentally spill anti-freeze (Ethylene Glycol) on an uncoated PC lens (especially on a hot day), it can and will literally disintegrate in minutes. PC can take a full swing from a baseball bat, but it can be crushed in your hand after a few minutes in anti-freeze. By the way, this is a great trick to impress the girls and wins some bets.
Bottom line is, if you use gentle detergents and keep the steel wool for cleaning scum off metal parts, your PC headlight lens will be happy for a long time.
On the question of Custom Dynamics... it sure does look like the same light. Since that lamp design is so unique, I'd say that you are correct.
Ride safe.