-What's the current thing with commas? Spice merchant doesn't have one before the "to", but other cards do. Baron doesn't have a comma before "for" but Mill does.
The current thing is to consider every card separately, and do what I like best there, tempered by whatever arguments LastFootnote makes, and randomized by not necessarily considering which cards are matching which other ones. Normally, I think "do X to do Y" doesn't need a comma; for Mill, I worried that people would misinterpret it without that comma (maybe you get $2 per card).
-Should numbers be spelled out? I see two is written out in develop, but it's "4" and "5" on Minion.
At some point numbers tended to be written as numerals in order to make card wordings shorter. I haven't necessarily been consistent there.
-Did the coin icons change? It used to be the number was smaller but in the hinterlands cards it seems to be the same size as the cost. Compare Minion and Haggler.
I'm not sure what you're saying, but the coins did change.
It used to be that the coins were a text number on a blank coin image. For Empires, they finally made a set of images so that the number was on the coin. This was to avoid endlessly saying "the 4 on this card's cost is off-center" (in previous years this was avoided by having numbers be off-center sometimes).
Also for Hinterlands the font on the numbers is different; note the $4 on Fool's Gold. The small coins now have the same font used on larger coins.
-Why do Treasures have a "When you play this". In other words, why was it necessary?
It seemed important back when to make it clear when the amount locked in, on treasures worth varying amounts. If you play two Banks, the second one is worth more. For other cards, it seemed both good to work the same way and thus be phrased the same way, and like it was a potential source of confusion if they didn't spell it out.
These days it feels like they don't need it. The variable amount ones could use +$ rather than "worth." And it would be great to drop it - it's so much text on those cards. But I didn't think of changing it at the perfect time and so everything still has it.