From Gravity Forms' own "Default Currency" setting (Forms → Settings) at the moment you build the form — that's the pre-filled default in the Build Recurring Payment / Build Payment modal, though you can override it per form to [...]

Two independent gates have to both pass. First, the currency has to support an instant scheme at all — currently GBP and EUR map to "Instant Bank Pay," and AUD maps to "PayTo"; every other supported currency has no [...]

No — it's the intended trade-off, though it's easy to expect the opposite. The "Upfront fee" field on the Create Payment Link panel only appears for forms that have a hidden wpdd_token field wired up, and that hidden field [...]