CubicsRube
Hero
Has D&D not updated Inspiration in an errata yet? I don't see why; advantage is usable in combat...
"Once per session" is an improvement over "DM will grant it to you" because it shows that PCs will predictably gain more Inspiration, and also that they will predictably lose the resource if they don't use it. I wouldn't force players to explain why, as it might deter them from using it, but I'd leave the opportunity open.
Any reason you left Alignment out of the IBFT family?
And from left field...how often do the Critical Role people use their Inspiration?
The explanation really I had in mind is just so they have an opportunity to flesh out their character in front of others. But really yes it should be very lenient and granted easily in practice.
As for alignment, it doesn't grant inspiration as per phb if I remember rightly, and I don't really use it in my games.