Qn 1. Usual case is that your main data store is MySQL, so Solr is the one that lags. One can either use the data import handler or write custom indexing programs to get data from MySQL to Solr.
Qn 2. Along with the fields you want to search (which are indexed fields), you can also keep non-indexed stored fields in Solr. This will help you build your data from Solr itself, without doing a secondary DB query.