Saeviomagy
Adventurer
Retries at knowledge checks require new information from an outside source (including the player figuring something out) in my game - you can't just retry them whenever your bonus changes.
Typically I give out XP/levels when there is a lull, so dinging mid-combat doesn't happen, and given that levelling up is a fairly disruptive process, I can't see any scenario where I might change that. If I was willing to level people mid-combat, then most of the rest of what you say would be true.
As for druids and metal armor? Who knows. As far as I can tell, there's nothing beyond "druids will not wear metal armor" written in the rules. No mechanical penalties and no flavour text. Just that one sentence. Given that, and also given that they use metal weapons just fine, I'd be inclined to say that the druid class doesn't give proficiency with any metal armor or shield, and leave it at that. A multiclass druid will have no mechanical problems, and flavour is flavour is flavour.
Typically I give out XP/levels when there is a lull, so dinging mid-combat doesn't happen, and given that levelling up is a fairly disruptive process, I can't see any scenario where I might change that. If I was willing to level people mid-combat, then most of the rest of what you say would be true.
As for druids and metal armor? Who knows. As far as I can tell, there's nothing beyond "druids will not wear metal armor" written in the rules. No mechanical penalties and no flavour text. Just that one sentence. Given that, and also given that they use metal weapons just fine, I'd be inclined to say that the druid class doesn't give proficiency with any metal armor or shield, and leave it at that. A multiclass druid will have no mechanical problems, and flavour is flavour is flavour.