Fill nipples:

If you look at Hydraulic fitting catalogs, Brass is only rated to 3000psi, carbon steel to 5000psi, and stainless can be much higher like 10,000psi. Technically, the prudent thing would be to use stainless for the added safety factor.

Most of the other hoses and lines people are using are downstream of the bottle regulator so they are only subject to about 800psi.

Can somebody please explain to me why SCUBA tank volume here in the US.....

Because in the US we like to do things our way. Some figured out how much air we stuffed into a cylinder and called it an 80. We don't like to change and we like to picture things in feet and inches. Doing things in BAR is like using the metric system we don't do it.

Why would you want to buy a converter and not a DIN unit in the first place:

You could certinally buy a reg setup for DIN from the factory and the price is usually the same as a yoke type. But given that most of the tanks in the US are yoke, as a diver you would want an adapter to go back to yoke for the flexability. Most charter boats and travel destinations don't offer DIN tanks.

Given that most SCUBA shops seem to charge between $4-5 per fill, I'd imagine that there is enough of a margin in there to pay for the higher costs:

Let me see $4.00 per fill, 10-12min compressor time at 220V/AC, $1.00 per hour for the filter, Labor, maintenance, loan payment, the list goes on and on. 30 SCUBA fills per week (small operation) that's $480 gross per month. No, there's not much margin in the cost of air fills. I'm lucky to break even. It's the sales made while the customer is in the store that cover the expenses and make the profit.