They stack with items, and are subject to the same caps.
(HCI/DCI/Regens etc.)
Invigorate intentionally won't heal through poison.
I'll have to revisit the code to see if casting focus allows you to exceed the cap. (I think it might.) It does definitely stack with the scribe bonus.
For Discord/Tribulation, the number of ticks ranges from 5 - 14 depending on skill level.
(Magic Resist factors in PvP and can lower this)
Preservance Damage Reduction will stack.
The Mana reduction affects upkeep of all mastery abilities (including discordance)
The timeout between skill use is 10 seconds. This is shared by all skills even bard skills. The timeout itself is just reduced when using the bard skills.
The poison resistance varies with the bard skill. So if your skill is high enough most lower level poisons will be resisted on the very first round. A 4x legendary bard will probably even shrug off lethal poison fairly easily.