Me and my partner have been together 15 years. We opened our relationship about ... 4-5 years ago.
1. My partner plays about 1-2 times a week. I play about 1 time a week. Some weeks it is 1, some weeks it is 2, some weeks it is 0. Sometimes we would like to play more but it does not happen due to the flakiness of people, other plans, etc.
2. Definitely. No overnights. Safe only. I am allowed to play with guys under 40 only who are preferably in shape. That was imposed by my partner as main of my reason for open relationship was to play with guys my own age. My partner is older. My own self-imposed rule is no anal first time I meet a guy. That is for my own safety and safety of my partner as well. I have broken that rule once (in general I would consider breaking it when I feel really comfortable and safe enough). My partner has the power of vetoing any one or any play if he so chooses. So do I for him. We rarely use it now as we know our limits and boundaries. Some rules may be broken. i.e. I've had overnights with a guy at a hotel room. I knew the guy. We never had sex though but we definitely horsed around

Exact arrangements depend on specific person, his specific relation and history to me, and my location at the time and location or my partner at the time. From what I have read, it is MOST DEFINITELY good to have restrictions and boundaries where you BOTH agree on them. Some other rules have been broken too, with trust, and care. Sometimes I got the heat for it, and sometimes the exception were approved by my partner. You must have trust. If you do not have that, then just break up. You both will have all the freedom you want then. The point of a relationship is that you do have boundaries and give up some of your freedom. Open relationship takes this to extremes and you have to be quite disciplined so not to get off the chain. With no trust, relationship is subject to slow bad decay, arguments, jealousy and general badness. Have a trial period (with strict boundaries). Your partner can have his freedom later, when he earns it
3. I think it was say .. 10-11 years? Yes. At first, relationship was opened up for me only and I had a 2 month trial period, after which the relationship was closed again. That is also when I didn't know what and how to be careful and I got a yeast infection, a cold sore and some other flack. Now I do not get those anymore. Maybe I built immunity or maybe I am being safer, or it may be my choice of people as well, but those things you may and will get even if you are super cautious. Just be ready to get some as well. Later, actually this year, my partner got scabies! Scabies, how archaic!! of an STD, ha... Good thing they are all curable. The number one thing to watch out for is HIV. Do not ever joke with that. Always be safe. Due to some specific kink and fetish I enjoy I got preventative shots for Hepatitis A and B. If you plan to engage into rimming or more, it may do well to get those as well.
Back on topic. After the initial trial period, my relationship was closed again. Due to things not working out sexually between me and my partner (I do not think we are sexually compatible really), we opened it up again for don't remember how long. Also, open for me only. Then we closed it again. We tried to rekindle our own fire but then, finally, about 2 years after I think we just figured that we might as well keep it open. And so it was. Also, it was still open for me only. My partner was at first "threatening" that he will open it on his side, I think it was if I was not satisfying him, he would open it. And eventually he did open it for himself as well. We have had it open on both sides for now about 1 year I think, maybe more. So yeah during those beginning times, my partner did not want an open relationship. Trial periods helped him keep this control. Until it became obvious that we needed to do something.
Even now I have an understanding that if I break rules badly, i.e. ignore our boundaries in a disrespectful and disregarding way, then I will lose my privileges of being open. If that ever comes, it will be negotiated as to what exactly happens, but at this time, there is no need. It is in my interests to stay within the boundaries. The boundaries I have are very VERY open, and I respect that and respect a lot what I have with my partner
4. So I do not know your situation. But, knowing what I know, it looks like your partner needs a sexual outlet, something he wasn't getting in the relationship (which is not your fault!!! or anyone's fault for that matter) and now with the new-found freedom, he is "off the chain!". He is like a puppy thrown in a new flower bed and he MUST explore it NOW and FAST.
In my case my partner is older. I longed for and desired someone my age. In particular, I could not get over my first love who lived in another country. I was eventually allowed to experience my desires. during my trial period I played with 8 people. Later I also did get to play with my first love. Both of which finally cooled me down from the "off the chain new puppy" syndrome. It took time though. When we first opened up our relationship for 2 months only, I had a deadline. I had to use my time and that time I was hitting up the bars, the Internet and websites really really hard. It was all I did. I had a deadline. Any slow-down would hinder my chances to use my freedom. In those first two months, after some time I lost my path. I began looking for Quantity instead of Quality. That did not give me much good (i.e. STDs for one). After some time eventually I figured that I much better look for Quality. I pass on guys now a lot more where before I would jump on anyone who would bite back who is barely passing decent. That said, I still sometimes jump on something these days but not nearly with same urgency and desperation as before. Still I have some days where I am really horny and my partner just not available sexually to me. So I spend an entire day on hookup sites and stuff. And other days I am more grounded and spend only some time on those sites or not at all.
So final words, without being able to talk to you in real time .. are:
About being nosy: You need boundaries!!! In my relationship, we openly discuss who each of us hooks up with. My partner even helps me out and on an occasion he books the dates for me (more so he books the dates for himself, but is more than willing to share with me, and sometimes has me in mind when he books the dates). You need to know your expectations. I know some open relationships rather not know who hooks up with whom. In ours we do know, or when we don't, that information is available upon request. Sometime we choose not to know, which is fine.
Some courtesy work will do good, i.e. planning to watch TV for 2 hours beforehand is better than say approaching him when he is busy Grinding and suddenly requesting his time without prior plans, when he could be in a process of hooking up.
If you have other questions I'll be happy to keep talking.