1. The simple message to communicate is that if a person is holding a toy or playing with that toy, it's 'their's' until they've moved on. For toddlers and preschoolers, I allow the child to use a toy for as long as they are interested (which often isn't very long); when they are obviously done, it's fair game. I do not make them 'take turns' at this age or set a timer, because it keeps the 'waiting' child distracted; instead, I encourage them to find something else to do.
2. Step in as need be. If you see your son pulling a toy out of another child's hand, then it's time to be his guide. ("Charlie is using this now. Let's find something for you/You may use this one".) If you see a toy being taken from your son by another kid, use the same language, keep it simple and brief. The big problem I've discovered at this age is that there may be some parents who want to 'let them work it out'.... as toddlers, this is an unreasonable expectation and the more dominant child will prevail. This isn't good for either kid involved in the situation. NOTE: it also helps to have at least two of the more popular items; in our toddler rooms with 8 kids, we made sure we had 8 hammer benches, 8 shape sorters, anything that is not a set of manipulatives (pop beads, blocks, duplos) we had 8 of. If you don't have identical items, consider putting those special single-player toys away, or ensure you have similar items so both kids can hammer or build or roll things--parallel play is how toddlers identify with other children. I offer replacement toys because the child's still-growing brain often perceives loss as real pain, just as real as if they'd been hurt. Knowing that their brains are still developing helps me to understand the big reactions some kids have when that toy goes away.
3. No-- if neither child is upset, I wouldn't step in. Let them cue you in as to when they need help, otherwise it's hovering and confusing. Letting them 'be fine' if they are fine is important.
4. Great question: empathetic language is key here. After handing the toy back to it's rightful user, if a child is upset, give them some understanding words: "I see you want the ball right now. You are sad/mad that you can't use it right now. You will be happy when you can play with it." and then I do try to redirect, if possible "Let's find something else to roll. Oh, look at this car. It rolls just like a ball." Distraction, distraction.... Bear in mind, too, that some playdates of anything more than an hour may be too much work for little ones, so if there are a lot of tears, it might be time to go home. Don't figure it as cutting your socializing time short, but just that some kids can't manage the complexities of this sort of play for long periods of time, which is why parents should stay close.
My other thoughts: this was the age that I actually opted out of my mom's group playtimes. With larger groups of kids, the parents can become easily distracted, and at this age, the differences in parenting styles begin to emerge. In my group, there are some more relaxed mamas who want to let them work it out or play out of sight at early ages than I'm comfortable with. I decided that the Mom's Nights Out were more important to me than the playgroups. There is a lot of emphasis on 'socializing' kids at earlier ages-- this really should be more about following the child's temperament and curiosity, not imposed on them. My son would freeze up at playgroup and climb me like a monkey and was very anxious with all of the chaos, so I knew this wasn't doing him any favors. He did much better with one-on-one playdates and now at five, does play very well with his friends. When you mention that your son is happy to take turns with adults, it's because *you* are also happy to take turns with him. Kids get better at 'taking turns' when they are around 3-5, and it's better if they have adult support when they need it, too.