I have a number of questions for decent Zerg players, as I've only recently bought the game and still haven't learned proper Zerg playing, but I can hold my own (until I get caught up microing and forget my base).

Anyway, first off, I recently did a 1v1 versus a terran player on a 4 player map of large size, and since the terran usually have a marine up before the ovie gets to the opposing player's main, I held my overlord back and waited for my 10scout. This wouldnt have been a problem, but my luck was that the scout hits all the empties before finding the enemy (ironically the one I would have sent my overlord at), by that time he had walled me off, so I had no way to scout him. Now, I have two options at this point, quick expand, thinking he is turtling, or go on the defensive, expecting an early aggression of some sort. I decided to quick expand, expecting him to be like every other terran player and turtle without expanding to islands.
What happened was the early aggression, I got raped by mass marines.
My questions for this scenario I suppose, are first off, what to do when your scout cannot retrieve the enemy's build, and second, how/when should I scout in 1v1 on a 4player map? Additionally, should I scout earlier than 10(building eleventh from extractor trick too)

Second, Photon Rushes, WHAT THE HELL MAN. I know how to watch for them, but what do you do against them?

Third, I got caught trying to defend against a triple gate (assuming he would zealot rush me), but instead he turtled and went for quick expands defending with his ridiculous amount of zealots (12 or so). I had been caught with my pants down, and thinking he could attack me at any minute with those zealots (he also began building Dragoo-I mean Stalkers by the time he expanded). In this scenario, what do I do?

I think that you'd never be able to tell... Even the pros have to pick an option and hope for the best.

I would've taken the xel' naga temples and ur opponents nat with zlings and try to contain before expanding. But it's really hard to say.

That Ovie can scout anyway against terrans. It has 200 health so it takes a while for the marine to kill it. You should be able to see something before you need to run unless you run right into that marine's position to begin with.

Photon rushes.. Either mass zergs when they start building them and destroy the Pylon or them fast, or build spinecrawlers. Other option is to rush to his base, which is probably undefended at this time.

In general, against fast-expanding opponents, shark roaches is the way to go because they are hard to pin down and can force the opponent to pay for dividing his army. Mutas can also harass. Hydras are pretty damn good against Protoss zealots/stalkers (And void rays).

No, if your too deep into the enemy base, the marine will kill the overlord as they are slow.

If you want to exactly know what they are building send a drone, or a changeling later on. I think the overlord is only good for finding where the enemy base is. Later on you can use a speed upgraded overlord to see the enemy base.
(and if they don't start putting turrets up, you can use the overlord and drop a lot of hydralisk inside their base.)

1) Scout with zerglings. Afraid the wall is going to hinder your information? Scout with a drone at 9. Your overlord should eliminate one base (assuming that is the incorrect one). Your drone now has two bases to scout for. Terran builds supply depot at 10, barracks often at 12, and another supply once the barracks is done, or nearing completion. This gives you ~1 minute to scout for both bases. If the terran lifts the barracks for a tech lab, often they're going to turtle. A barracks constructing a reactor often leads to either a hellion harass or siege tank/marine push.

2) You scout a photon rush. If you don't you suck. Worst comes to worst, lay down a creep tumor before he can build a cannon in your base. This will spread creep and prevent anymore cannons from being built. Once you have contained and killed off the rush, Zerglings eat bases.

3) Mass zealots means you can easily go Roach Hydra, Mutalisk or mass expand yourself. If you're not going for a one-base build, expand before 30, and again at 100, continually as needed.