I don't disagree; it's well-settled one can hold 2-handed weapons with a single hand. My issue is whether "propping it" in one hand counts as your free action for the round. Absent a rule, it's pure DM discretion.
If so, then for the rest of the round it would affect your ability to take attacks of opportunity because you'd have to wait until your next turn to use a free action to get 2 hands on the weapon again.
If not, the Warcaster Feat, subsection #2, becomes irrelevant when using 2-handed weapons.
Also see D&D's
2015 "Rules of Spellcasting" (Sage Advice) page, relating to when your hands are free for casting:
There's a specific rule under Material Components that says when a hand is holding a spell's Focus, you can use the same hand for Somatics, but this ruling is not extended to spells without a Focus. The cross-reference is whether there is a "spirit of the rules" that a restriction applies unless you take the Feat to overcome it.