1) Yes you will be still be able to use it, you'll have to configure your Netgear Nighthawk as an access point and connect it to your main Viva router through an ethernet cable, that's all. I know this from experience and I'm assuming you're using the R7000 router, I own the same.
2) Yes, you can use any sim card compatible router, all you need to do is insert the sim card in the router, the router itself will grab the configuration of the operator automatically. I did this recently, I own a Zain Huawei E5186 router, cancelled my subscription with Zain and got a data line from Viva, all I did was pop in the new SIM in the rotuer.
Let me know if you have other questions.