Hello,
I have read all your requirements, You are looking after a "React Website Developer". I could work with your project to fix an issue with it and i have read all your questions and answers are given below:
1.) Yes it's possible we can do integrate React JS in WordPress.
2.) Yes it will work.
3.) One database will work here.
Apart from this, i have been working as a WordPress developer for 8 years. So, i could regard myself as suitable for this work.
I am waiting for your reply to proceed further.
Thank You.