How to integrate Elasticsearch in .NET?

Question

ASP.NET

There is a project on ASP.NET MVC 4, SQL Server via Entity Framework. It is required to add full-text search, the choice fell on Elasticsearch. Previously unfamiliar. I've been studying for a couple of days, it seems like a good thing. The problem is this:
1. How to correctly add Elasticsearch support to an ASP.NET project, the base of which is on SQL Server? How is the issue of synchronization solved? While searching for answers, I found a certain "river" plugin, but, it seems, it was abandoned, because it is evil. I did not see any other alternatives to synchronization.
2. As I understand it, Elasticsearch indexes only those fields that the user specifies. It turns out that there is nothing wrong with having a copy of the data in both SQL Server and the Elasticsearch database?
3. Are there other options besides Elasticsearch for .NET?
Please share your advice and opinion. And, if possible, immediately with links to projects, examples and good practices. In order not to cut a crutch, but to make it more or less ashamed.
Thank you.