The date and kick-off time of the two remaining quarter finals will be announced after next week’s fifth round replays.
http://www.thefa.com/news/2018/feb/23/fa-cup-on-tv-quarter-finals-230218
From what I can tell PEH has played in one final, of which he was substituted when the game was 0-0.
Caceres has not played competitive football for over a year. Playing him ahead of Stephens would be idiotic.
A bird in the hand is better than two in the bush. I'd buy one tomorrow, but select perhaps less desirable seats (back couple of rows) and hope that nearby seats will be available to +1. Probably touch and go if they're available at that stage though.