As a new manager, the biggest transition you have to make is going from an individual contributor to a coach. Your success will now be determined by how well your direct reports perform. You can't do the work for them, but you can teach them how to do the work well.
1. Since your boss's boss likes you, ask him for who else you should meet in the company. For meeting people outside the organization, go to industry events and try to form relationships with your peers at other organizations. Again, you can ask your boss's boss which industry events you should go to. I wrote about it more in depth on my blog: http://www.climbuptheladder.com/networking-more-than-linkedi....
2. The best way to overcome people unhappy with your promotion is to make them look good with their bosses. When they do a good job, let their boss know what a great job they did. If these people report to you, let your boss know that they did a good job. Over time, they will be less threatened by you when they see that you are on their side.
3. If your manager is scared that you'll replace him, you need to show him know that you're on his side. Again, when he does a good job on something, let his boss know. Ultimately, show your manager that you have his back so he'll feel less threatened.
4. If you are managing a team of 8 developers, you'll likely get a little bit of time to do technical work occasionally. You will certainly spend a lot of time helping the junior people who report to you with their technical work. Other than that, you can try doing some side projects to stay in touch. But other than that, remember that you are no longer an individual contributor so technical skills aren't going to be very important for promotions in the future. You need to demonstrate that you understand how to run a successful team and that you understand the business side.
I wrote about how to be a great manager on my blog which could add some more insights: https://www.climbuptheladder.com/how-to-be-a-great-manager/.