Thanks for the questions! Here are my preliminary thoughts:
"- Detect Evil/etc spells/abilities: are they sufficient evidence to condemn someone?"
They would be sufficient evidence to watch someone closely and to warn others that can't detect evil - politically it might cause people in authority to remove someone from a position of responsibility. But it would be evil actions that are condemned as far as putting someone in jail or attacking them goes.
"How are false readings (deliberate or not) handled?"
If there is reason to believe that a reading is false, then evidence could be given for this. Deliberately cloaking one's evil nature would indicate that the cloaker was up to something. Accidentally registering as evil when on is not would require investigation as to why a false positive is being given off.
"- You can't lie, but can you twist the truth? Lie by omission? Simply not say relevant things? "Hey Evil Guy, you're about to walk into quicksand." "
I think the LG guys would not engage in deceptions like that, and would be likely to say relevant things if not in combat. If in combat, they would focus less on talking and more on fighting, so omissions would be more allowable.
"Must you announce your presence & engage the enemy on even terms in the open? No setting up an ambush or scenario where you have the advantage? Are all combat actions available to you -- disarm, sunder, trip? Can you use a spell to curse or transform someone, possibly ignoring them afterward?"
In combat there is more leniency (so disarm, sunder, trip, feint, are ok). Similarly with spell use for spellcasters. But before combat they would have to be more honourable, so setting up ambushes would not be allowed, and announcing one's presence and engaging the enemy on even terms would be encouraged (assuming the enemy would follow suit - one doesn't have to go out in the open if one expects that the enemy would be dishonourable and simply have their soldiers fill one full of arrows).
"If someone begs for mercy, must it be given, even if you suspect it's a trick?"
Usually yes, although that could mean "strip them, tie and gag them, bring them back for trial". It could even mean "turn yourself in to Lord X, and if I see you again that means you don't get a *third* chance".
"Can you act on intuition & be proactive, or must you always wait for Evil to actually make a move first & be reactive only?"
Depends what being proactive means. You can be prepared for the others' actions, and could challenge them, or gather information on them. You couldn't kill them just for registering as evil, they have to do something first (just as cops can't).
"Can you end someone's misery, given there's concrete evidence of a pleasant afterlife?"
If they ask for it and are in their right minds, the misery is unbearable and a fate worse than death, and you don't have a way to relieve the misery.
"Is resurrection frowned upon?"
I'm not sure it will be possible in my campaign. If it is, it would be allowed (but the person might refuse to return).
"Must every life be spared, or can you kill evildoers?"
If they don't ask for mercy, or screwed up their 2nd chance, then you can act more harshly, up to and including killing *doers* of evil.
"Sacrifice some innocents to save many?"
Nope.