no mount effects your fall damage if you aren't actively riding it... I don't think any mounts reset fall distance if you equip and dequip them in the same fall, and it's a misconception that fall damage is based on velocity... it's based on the last place you used a jump or contacted a block or liquid. It's a relatively well known bug that if you jump right before using a teleportation potion, if you don't land in water or land exactly on the ground, and where you teleport to is significantly lower in the world than where you started, you'll die from fall damage.
no other boss is spawned by doing damage to a critter... no other boss spawns on top of the summoning item immediately after use... no other boss instagibbs (minus dungeon guardian). Maybe it's trying to hint to you that there's no way you'll ever beat this boss with a melee pure without zenith.
4: Is your smart-cursor on? smart cursor uses an interactable entity priority system to determine what is interacted with when it's not obvious.
Interactable entity(IE From now on) order is a very inconsistent system when the mouse pointer isn't directly over the action box of an IE that's also within reach (smart cursor on). there are factors to do with it that include your current x position (specifically whether it's even or odd) and the same for the mouse pointer. that's before you account for user error- maybe the chest or NPC wasn't within reach and the portal was the only IE within reach (or the closest* one in the direction of the cursor)
* closest in this context is also somewhat inconsistent. the interaction will default to the 'first furthest' IE.
first furthest means the first block the hypotenuse of the right triangle -made by your feet and your mouse- passes through in the action box of the furthest IE within reach(deep breath), even if much more of that line passes through another IE at the same x (which has a block you can reach but the line does not pass through this block), and that's before you include overlapping IE's like the money trough and the portal frame (which, btw, don't have a defined, obvious, x coordinate in blocks tied to their action box- a 2x2 item which can occupy a 3x3 space partially), or walking NPCs and chests (who hasn't had this problem) (all of this is my own 'speculation' based off of testing in 1.3 and 1.4. I'd love to have the time to look at the code for stuff like this but alas. only so many hours in the day and priorities must be made)
Mind you, I'm not saying that system is good or I approve of it or I don't think sometimes it behaves anomalously... Just that it's behaving basically as intended. Can't tell you how often I have accidentally removed items from item frames I use to label chests and levers instead of opening the chest or lever because my mouse was 1 pixel off or the chest/lever was 1 block out of reach.
