Ultimately, your XP gain = Enemy's HP x Difficulty Modifier, so it doesn't matter if you kill the enemy quickly, in 2-3 hits or shots, or take your time. As for Strength:
"Strength increases *max* melee damage by 1 per stat point (from -2 to +4), so if a melee weapon’s range is 4-10 and you have STR 8, then your damage range is 4-12, not 6-12 the way it works in most games, including AoD. If your STR is 4, then your damage range is 4-8. Thus STR gives you potential damage you may or may not roll rather than guaranteed +X to whatever you roll. Before you start complaining, high STR unlocks a feat reducing your attack speed with two-handed weapons, so it’s not a dump stat."
Using AoD as an example, you can have a heavy fighter who hits once with a two-hander for 11-14 points of damage (power attack with a two-hander) and miss 2-3 out of 5 attacks on average or a fast fighter who'd hit 3 times for 3-6 points of damage (9-18 points of damage per turn) and miss 0-1 out of 5 attacks.
Well... imagine fighting Antidas in his palace (the third IG quest); imagine doing nothing for 2 turns to farm XPs. Odds are, by the time you're done farming, half of your team will be dead and the battle will be lost.
1.
Can't save in combat. Anyway, the idea - as always - is to keep combat challenging. By the time the fight is over, after 3-4 reloads, you should be happy that your party is still alive somehow, not thinking of doing it again to gain extra 10xp.
Not planned.