In my experience Holy Light seems to crit more than Flash by quite a bit, but of course it could be in my head because I haven't done any longterm tests.
Holy Shock used offensively never gives back mana when it crits due to Illumination, only as heals. Here's an example of my combat log(Hitsmode) when I just tested it. I used Holy Light to lose 660 mana.
[10:25:11] [You] Holy Light [You] +2784
[10:25:11] [You] gain Light's Grace
[10:25:13] [You] gain Divine Favor
[10:25:15] [You] Holy Shock crit [Druse] *706*
[10:25:15] [You] Divine Favor fades
[10:25:26] [You] Light's Grace Fades
(notice no mana gain)
However, the most interesting bug that I've found on my Paladin is that the Holy Guidance talent (35% of Intellect added to Spell Damage/Healing) in fact also adds 35% of your Intellect to your weapon damage range on the character sheet. This is why Paladin's running around in PvP with one handers in tier 3 are still hitting people for 3 digit numbers even with 300 attack power and 41 dps healing maces.
With my Scepter of the False Prophet this proves to actually be quite a DPS increase, the faster the weapon the better since the damage added per swing is the same regardless of speed(Not that your trying to DPS as this spec and using this gear).
(notice that 35% of my intellect is being added to my damage range)