Hi @alkexr, thanks for the update!
Hope things are going well for you and you can spend a bit of time on the map 
One question on the removal of mechanical variety. You are planning to remove flanking vs ranged but you want to introduce/keep/adapt charge vs. spearwall. This seems a little weird to me. I think I understand where you are coming from but charge is very similar to flanking in that it's mechanically different but not strategically (in your sense of the word).
"Ambush" seems like an interesting idea to add a strategic option. It seems to be delivering what you want to achieve. "Scouting" on the other hand has the problem that there is no upside to it. It is purely a reaction while ambush has no downside. (And please don't buff the elven rangers even more by giving them another ability, they are already arguably the best unit in your map.)
Retreating heroes might work well. I don't think it will make bringing a mage or a balrog the right move but it can make it less of a bad investement (in particular if the balrog can finally affect the foes it will actually be fighting). If dragons can retreat like submarines, though, they will be way too strong/unkillable imho.
The changes to charge seem a mixed bag. I might be thin ice to comment without having seen/tested the prototyp but ithe new charge seems overly complicated for a mechanic that was very weak and easy to counter in the past (the targeted attack against them meant "charge" was a liability and it was better not to build chargers at all in many circumstances). The new charge is a lot stronger just by not making formation/spearwall a targeted attack (excellent change) but the rest seems very hard to keep in mind/consider when doing some rough calculations. It just does not seem to add much in terms of depth to balance it out.
Siege was/is an excellent ability from a mechanical point of view imho. I would not make it weaker in caves and so on. It just adds complexity without much benefit. It seems this is targeting very niche scenarios or no scenarios at all.
I hope you had a chance to look at my explanation on how to simplify the user interface/nomenclature for terrain effect and "tactic" modifiers without loosing any functionality. Some house keeping would really help on that front.