I think the 2 moves to cross rivers would also require new code -- afaik rn you can have it cost 1 move flat (no matter the terrain on the other side), or end turn, no in between. iirc embark/disembark can be given varied flat costs in this way so if you want to have a "staged" ability, costing less over time, you could focus on that one.
I can't comment on what may or may not be implementable in dll as new features, just assessing based on how tables & abilities work currently. Simple sounding things are not necessarily simple to adapt into the .dll, I can say that much. To get sponsorship on a proposal it will be best to focus on what can currently be accomplished, though I do agree that it would be nice to have more customizable movement rules on a per terrain/feature basis. Unlikely to happen however, this was effectively what prev recon proposal failed on. Anyway most of what you've proposed looks achievable, and possibly an improvement over status quo, though admittedly I have difficulty imagining how broad changes like this will play out in-game, til I try them; it may be a worthwhile experiment, with some small revisions.
Existing features (or lack thereof) aside, the only red flag I see is giving recon attack and move -- imo this is the defining feature of mounted units. I just don't think it fits with a high mobility infantry unit, outside of maybe UUs. I don't love the idea of a pathfinder with 5 moves either; it's too fast, I'd say we want a fully promo'd recon to be no faster than the base moves of a mounted unit from same era, so for pathfinder max 4.
On the other hand, I do love the idea of using VP's plague feature ie your "disabler" promo, however as it stands I don't believe a plague can be applied during attack only; it is applied via any melee combat. Attack-only plagues would be another nice-to-have feature in the dll, but not there rn (or at least I don't know how to make 'em work this way using tables alone). It CAN be done in lua for modmod, but generally lua tricks are out-of-scope for mainbranch VP. Recon overhauls that use plagues to make the line unique do raise my interest considerably, I think there's lots of potential in that direction, especially since 3.8/9 just improved how these work (according to patch notes anyway, have not tested)
If mountains can be traversed then they cost 1 movement.
I'd qualify this by noting that this is only true for a "ignore movement cost" unit -- mountain access promo without ignore results in unit ending turn for each move into mountain. Anyway I believe 1 move cost in mountain was the intended result in the theorycrafted proposal? maybe I misunderstood, but that one should work as I read it.
Something to investigate/test that may be useful here (or someone can comment if they know) -- I'm not sure how ExtraMove combines with ignore terrain cost: it's the same table as double move, which stacks on top of ignore; if it stacks with ignore as well, we might be able to slow recon down selectively in some plots. If this is possible then I don't mind the proposed high movement point recon as much.