Hi
I tested recently Apachae Solr 4.9.1 with AEM 6.1 and it works without any problem. I did not have any problem with that version.
To your second question I have no answer, I am sorry. To the third question, I think it depeneds for what you want to use Solr. I know that AEM 6.1 has a integrated Solr Server but it is not the same as an standalone Solr instance. I am using Solr for UGC with Mongo in the background and it works great. Adobe also recommends to use Mongo or Adobe Cloud for data consistency.