Holy Light should work as follows - Any undead that can attack the Holy Warrior that round take damage from it if the Holy Warrior's attack succeeds (damage is per undead, not divided up among the undead). Any undead that cannot attack the Holy Warrior (because of maximum # of opponents, or because of a shove) are not hurt. Though undead are hurt by it, all other attacks should be judged as normal, even if the undead attacking are doomed to be destroyed by the Holy Light that same round.
In my opinion, a fighter should be able to use more than 1 shove in a round, but doing so of course uses up their shoves or their battery. In YBA 2.0 I might very well consider making shove slightly more expensive as it does seem to be very useful in a solo adventure.
If you are shoved first, the only action you can take is to shove someone else.
It is also worth noting that in any round, a person playing a dirty trick first cannot be shoved but a person who is shoved first cannot play the dirty trick. Furthermore, if a person does a chair shot and is then shoved, the chairshot still counts as it is an immediate effect.