I think Gem often uses the same method as Graham Coxon for Barre chords, which is the way i often do it too. Use your 4 fingers for 5 strings, barring the 2 thinnest strings with your first finger..but then your thumb over the top over the low E to fret the lowest note. I believe Hendrix aways used that way too.  It's a bit weird at first because your thumb placement would usually be a bit further forward than the note you need to fret, so you have to re-adjust your thumb back and then put it over the neck.

It's probably harder work than barre chords really, but i think it just depends how you learned to play. I grew up learning Jazz guitar and it was often a common method so you can keep a chord and melody going but switch bass notes with your thumb.

yes. of course, you have to understand what a capo does and why it is used.