Questions about how experience is gained

[DGDobrev]

You do get for summons. You get 0.017 XP per point of leadership, regardless of the stack, and regardless of your army composition. This is why I'm doing a test playthrough with max Black Knights to see if bosses can be exploited this way

As for the XP gained for battle, this is what modifies it:

Learning - up to 20%
Hand of Necropolis (gloves) - 10%, game can spawn 2 of them
Memoirs of the Marshal (artifact) - 5%
Shark Tooth (regalia) - 5%
Chain of Chance (belt, with gift bag) - 5%
Treasure Searcher medal - 7%

so in essence, you can get up to +62% XP per battle.