Until I was made redundant, I worked in Orange. My route was the 405 South (note: “the [freeway number]” is how we say it here) to the 22 East. If you’re looking for Little Saigon, then that would be the Brookhurst exit. Try My Nguyen (“May NGwen”). I like the Bahn Xaio, and many of their other offerings. There is a Harbor Blvd. exit off the 22 which is a few miles south of Disneyland.
If you want to go to Disneyland, then 405-22 is a good way. You can also take the 105 from LAX to the 605 South to the 91 to the 5 and get there that way. It’s a little shorter, but traffic can be bad. (Actually, traffic can be bad anywhere – especially on the 22 East in the morning, the 22 West in the afternoon, and the 405 North in the afternoon.)
To get to Hollywood from Garden Grove, take the 22 East to the 5 North to the 101 North. From LAX take 105 East to the 5 North to the 101 North.
I’ve driven around Sunset in the wee hours. Not so bad driving, but there can be some rough trade. Hookers and street people. Probably harmless, but as silenus you need to know what to look out for. Santa Monica Blvd. in Hollywood abounds with male prostitutes. I don’t know how aggressive they are in general, but I was propositioned by a male street person. I told him I’m straight (actually, “I’m not into guys.”) and he said, “Oh, but you have to be!” Kind of amusing. This was in the early afternoon, BTW.
I’ve driven around downtown between 0100 and 0500 (I used to go to the now-closed Gorky’s Café). It’s fairly empty except for the street people. Not a place I’d hang out after dark. There are some iffy neighbourhoods right next to downtown. Be careful.
Also be careful about getting off the freeway in South Central. USC is down that way, but I’d avoid getting off at Hoover, Vermont, Western, Normandie, etc. in the late night/early morning.
I’ve never felt unsafe in Hollywood at night. But then, I’d go to The Cat & Fiddle or House of Blues and then come home. You’re safe enough in your car. I don’t want to scare you. Just be a little cautious.
An old copy of Let’s Go: California said this about hitchhiking: “Hitchhiking is tantamount to suicide.” They don’t put it that way anymore, but they do say it’s a remarkably bad idea. If your car breaks down, don’t ask a random person for help. You might flag down a police car or use your cell phone to call for help.
I’ve never been to Mexico City. Isn’t it the biggest city in the world? You are familiar with the problems there, and L.A.'s problems are probably similar.