2) Plot can occur in a sandbox campaign and it is best implemented as a series of events written as if PCs didn’t exist. Consider this as a plan that will change once the PC get involved. They also may not want to be involved so plan for that.
In the Daen Ral setting that I've worked off and on for the past like 20 years I have, at times, tried to completely detail areas (almost to the point of knowing what everyone had for breakfast the day that the PCs encountered them...) that I thought would be interesting for the PCs to play in...but I've always had a kind of sandbox approach to the game - so the players would always latch on to some throwaway comment that I made about "the north storms" or the "power gathering in the Midlands" or whatever - and they'd go chasing after stuff I never thought of...in places I hadn't detailed. So I've used a ton of published adventures with "the serial numbers filed off" as it were back in the day - in fact, at one point I was hauling my big stack of hardbacks and anywhere from 20-40 dungeon magazines and adventure modules to my friend Jim's house - "just in case" the PCs took off in some unexpected direction (like when they were trapped under the ruins of the ancient elven city (using DL1 Xak Tsoroth for the ruins) when they (in their 4th level glory) ran afoul of 2 demons (Type V maybe? don't remember - never have scaled adventures much for the power levels) when the paladin was at zero hp, the F/MU and the T/Ill were in single digit hp, the MU was out of spells, so the cleric prayed to his god for deliverance - something to the effect of "deliver us to safety" or some such. I reasoned that he had about a 2% chance to be heard (I was so ready for a TPK) so I told him to roll d00 and if he got 99 or 00, he was heard. He rolled a 99. I hastily built a chart in my head of possible "deliverance" scenarios - rolled a die and they found themselves, in a brilliant flash of light, in a desert oasis... Eventually they found their way through about 2 pages of the Al Akbar module, then in the lost city of the Cynidiceans where they crippled the cult of Zargon by collapsing the temple on nearly all the priesthood (though they never even came close to destroying the creature himeslf) leaving the city in the capable hands of the party's cleric and a subdued/befriended blue dragon...
Anyway, the point - I guess that with enough material at your mental fingertips play can continue pretty much uninterrupted if the PCs go off in some unexpected direction.
So how all this relates to Rob Conley's quote...um, well, maybe not well. Here's the thing: I've always had these kind of background plots running - sometimes big, overarching, world shaking plots (the raising of the dead gods, or whatever), often kind of local scenarios that maybe the PCs completely derail or help along or whatever.
So...in Daen Ral I'm trying to detail the area around Jacob's Well and I keep going back and forth about how much detail to include and I'm thinking about doing a sort of general hex by hex description (hex-crawl, I think I hear people talk about) but on a more micro level - that is, the hexes will be maybe 1/2 mile across - but each hex ought to have something interesting in it (maybe a stand of trees burned in a recent lightning strike or small humanoid tracks or something) and then the major encounter areas can be more fully detailed (the Temple of Darkness, the Wyvern's Lair, Hope Cross village, Seeley's Glen, Hag Island, etc).
So the plots come in with, dare I call them, Adventure Paths - hmmm, maybe Adventure Tracks...yeah, that's more Old School...heh... Or maybe I don't have to call it anything...
Here's the deal - the adventure "Darkness in Hope Cross" involves the village of Hope Cross, the Southdowns, a section of Seven Stone Creek and the Temple of Darkness. If I have a general overview of all the hexes in the region in the Jacob's Well "sourcebook" (for want of a better name), then I can include in this "adventure whatever" just the areas directly detailed for the adventure (some special notes on some Hope Cross residents, what the shepherds in the Southdowns know about the situation, the Temple itself). But any hex not indicated in the adventure is fair game to be explored - so the party could still come across the Wyvern Lair (from a different adventure - A Question of Balance) or the Ettercap lair (from the Mysterious Disappearance of Silian Jass) and do as they may for those encounters - if it throws off later "adventures" so be it - maybe a few notes can be added in the adventures about what to do if the PCs have dispatched the Wyvern, for example, or found Silian Jass before they knew he was missing.
Heck, if PCs just want to wander around the area and discover things, that's okay with me too. There's plenty of NPCs, ruins, hidden humanoid outposts and the like, not to mention a few monstrous lairs and such to keep PCs busy enough even without "adventure paths/tracks/trails/hooks/notions/whatever".