I doubt you will get a useful answer in a comment. Each of your questions is very broad. And you didn't even select a search engine yet. You didn't specify the scale you're dealing with. You didn't specify the number of reads/writes per second that you expect.
Choose one system and learn it well.
I recommend writing down what exactly scale means for your needs. Number of users? Number of queries? Number of sources? Number of 'result-sets'? Number of documents? Number of text fields?
Elasticsearch is built on-top-of Lucene which is a Java API that you can use in pretty much any application. If you already have a system that can search the MySQL clusters then I would recommend hooking Lucene into that system instead of standing up another one.