2a. This is possible in the case of Knights. However, there is no effective early-game strategy on the Stronghold map with Knights, so the requirement for the Stronghold is not necessary and is a matter of taste. On the other hand, the Stronghold emphasizes the elite nature of the Knights: stables, which are available in any town even without a Stronghold, are accessible everywhere; while stables that come from Swordsmen reduce the variety of already rare early-game strategies involving Knights.
2b. In the case of Griffins, a ranged unit is mandatory; otherwise, the combination of Stronghold, Griffins, and Angels in the first week is imbalanced (too advantageous). In general, Griffins should not be underestimated; 2/3 of effective early-game strategies go through them, while only 1/4 go through Knights.
3. This is a controversial issue: opinions are divided; some prefer Archers from Militia, while others prefer them from the Stronghold (the survey is not only conducted on this forum). Is it correct to be able to place Archers in towns so easily, bypassing the Stronghold, or should the Stronghold be more important in this case? The final answer will be provided by practice.
4. Yes, there is no point in upgrades here. For training, the effect of +10 is already more than enough; the second building in the same category is unclear why it is needed. The same applies to the improved Altar; it does not provide any fundamentally new possibilities.
Regarding the comparison of Griffins and Hydras, I would like to add 2 points:
1) Hydras are a different faction that starts without Tier 1, so the buildings of the Dungeon should be slightly stronger overall.
2) Hydras are now fundamentally less demanding but more late-game (they can only be built on day 5, while Griffins or Raptors can be built on day 4, but обязательно through the Stronghold).