On the first issue, I'm not of much help because my two still do it, also. If I could get everyone I know to ignore them, leave the house, or demand a sit, I'd be all set. I've had people start to come in, then as soon as they get crazy, leave the house. We did this exercise for several minutes, but because I can't get everyone to do it, it doesn't really work. They are both very well trained so this is what I consider our one issue and I've decided to manage it, not fix it. So, if someone is coming over who doesn't like or can't handle them, I either crate them or put them in another room or even in the fenced yard before they come over. I only put them in the yard if someone is popping by for ten minutes to drop things off. The peeing is just an excitement thing. If someone comes over, make sure he's just peed and that may help. Neutering is not going to help this. It's not as if he is marking.
As far as the pulling, some will recommend a properly fitted prong collar. However, he sounds like he might have some submissive tendencies, so I'm not sure if that is the answer. There are some harnesses that people like as well. I personally like the halti or gentle leader, but you have to slowly get your dog used to it because they generally hate it at first. Also, some people have issues with rubbing. I never did, but I suppose if the fit is off, you might. I don't allow my dogs to sniff on walks unless it's time to go potty. I give them several potty breaks where I'll say, "OK, go potty", and they'll sniff around and go. I only give them about 15 seconds though so it doesn't turn into a sniffing frenzy. Over time, they have both come up with ways to let me know they have to go or I notice they are walking funny or looking anxious, and then I make it my idea to go. They are both nearly 100 lbs and young and I am a 5'4" woman. I walk them both on flat-buckle collars. Two in one hand. Linus gets to the end of the leash but Sam's is slack. I think training is the answer, but most people with adult dogs go for a device.
Your final issue with your dog shouldn't be an issue at all. You say your dog "absolutely loves laying on his back with his paws in the air" so why would you want to change that? Or is he actually being submissive? What is the difference between petting a dog laying on his belly and one laying on his back? Unless the dog is being submissive -- and in that case, change will need to come from you, I just don't see that this is the hill to die on.