You nailed it. You've perfectly deciphered the core separation in the game's engine: The Ledger (The Math/Capacity) vs. The Inventory (The Physical Cards/Flavor).
To confirm your exact points:
1. The Green Ledger (Your Capacity) Yes, your understanding is 100% correct. You start with a Ledger capacity of 3 Green draws. Unused Green capacity rolls over from year to year. And, crucially, if you pay the cash penalty for unbuilt Blue steel at the end of the year, that unbuilt Blue capacity converts into additional Green capacity for the next year.
2. The Green Cards & The Parlor You are spot on again. Buying a new Green card at the Parlor adds a physical card to your discard pile, but it does not increase your Ledger capacity. As you rightly pointed out, buying Green cards doesn't increase your ability to build track—it changes the distribution and quality of the bonuses you receive when you do build.
3. The Carousel & The Discard Tax This is my favorite part of your comment, because you've stumbled onto a major strategic layer of the deckbuilding!
Under the hood, this is called the "Paradox Rule." If your Ledger demands Green steel, but your draw deck is empty, the conveyor belt pauses and instantly recycles your discard pile to keep feeding you.
As you noticed, this heavily messes with the Blue "Asset Surcharge" contract (which taxes you based on the size of your discard pile). This is completely intentional!
- The Fat Deck Strategy: If you buy a ton of Green cards at the Parlor, you get more, varied bonuses. But your Green deck will swell up, making the Blue Asset Surcharge card more dangerous.
- The Lean Deck Strategy (The Wild Combos): If you never acquire new Green cards, but instead spend your Parlor visits upgrading your starting 4 Green cards, your deck recycles constantly. Because it's so lean, you can reliably trigger Level 3 effects multiple times a year. And, as a bonus, your discard pile is tiny, mitigating the threat of the Asset Surcharge tax.
You completely cracked the underlying math of the engine. Thank you again for digging this deep into the systems!