It sounds to me like two things are going on:
He's reacting to the new baby (it sounds from your post like you've got a baby, a 3 1/2 year old and a 5 year old -- that's a handful for you, no matter how well behaved)
He's wanting a bit more freedom than he's getting.
First of all, I suspect he's annoying his brother and the baby because he wants attention and he knows it'll get him attention. The antidote to that is to take time every day (not sure how you're going to work it, but maybe when the younger two nap?) down on the floor playing with him. The key part to this is that you have to let HIM direct the play for these 20-30 minutes. Have him choose the activity. You're the hired help and play along. You don't direct. You don't tell him how to color, drive his cars, or how to react. You can make your characters act, but watch his reaction.
A great book that describes why this works is called Playful Parenting -- basically you're filling his cup of attention, and you're seeing what's interesting to him or bothering him or on his mind. Right now, it's pretty clear from my kids' play that ds is intrigued by the idea of male aggression, especially when playing sports. When we play baseball, he'll 'charge the mound' if a pitch goes close to him. With basketball, he'll argue loudly with me about fouls, or he'll foul me blatantly. Now, ds is possibly the most mild mannered, least aggressive kid I know. He's a rule follower. But playing baseball/basketball with me gives him a safe space to see what it feels like to break the rules and be aggressive. (OK, I'm sure the neighbors think we're both completely nuts, because it does get pretty loud!)
For the toys - teach him to trade. Maybe set aside a couple of 'special toys' for him that he doesn't have to share with his younger siblings. But then if he wants something of theirs, he has to trade them for it. With the baby, it'll be easy -- they'll take anything. With the 3 year old, it'll be some more complex negotiation, and he won't always get his way. That's OK. Let him have his meltdown, comfort him and move on.
For the 'running away', is there any way you can meet your neighbors who have kids? If there is anyone with kids near his age, and it's only a few houses away, I'd be OK with him walking there by himself and asking if the kids can play. You could start by calling them neighbors and asking if they can play, then sending him down for an hour. If he just wants to play on random people's lawns, then yes, the pp is right, you have explain to him that people don't like to have their stuff messed with.
When you're out for a walk -- give him small boundaries and see if he can meet them. For example, he can walk up 2 houses and wait for you to catch up. If he can do that, then maybe he can go to the corner and wait for you. At 5, my kids could go about a block ahead of me (if I could see them), as long as they waited at the corner. Usually what happened is that they raced to the corner, and then raced back. Make sure you're phrasing this as what he can do and not what he shouldn't do. So, "don't run" isn't nearly effective as "wait for me at the corner".
Bedtime -- what I would suggest is (a) starting bedtime 30 minutes earlier and (b) expecting to take longer with him. Again, the coming out again and again suggests to me that he needs attention, and this is a sure fire way of getting it. When kids are feeling like they need more attention, they'd rather have negative attention than no attention at all. So, what would happen if you sang your song, and just sat in his room with him chatting for 10 minutes? Then leave, and say "if you stay here, I'll check on you in 5 minutes". Then do it. We used to set the timer for ever increasing intervals. We'd check on the kids at 5, then 10, then 15, then 20 minutes, etc. After the 10-15 minute interval, they'd usually be asleep. Sometimes it took up to 30 or more. But the checking on them helped them feel connected. The timer reassured them that we wouldn't forget and gave us an incentive to give them to stay in bed. (We'd reset the timer if they got out.) Would a radio on with some soft music help him feel less 'lonely' in his bedroom? Both my kids have radios. Our daughter listens to music, our son to sports talk radio.