It would take a book to answer all of these questions. In fact, consider buying Mastering Digital Printing, Second Edition, by Johnson. This will answer all your questions, and then some.
But quickly on the points that you've raised: throw away Printfix. It's a toy, and will produce more problems than it solves.
Use the Epson supplied profiles, or better yet have a custom profile made for your printer, paper and inks. These cost less than $50 most places.
When you print, use Print with Preview, turn Color Management OFF and use the intended printer profile.
DON'T PLAY WITH SLIDERS!!!!
When you shoot a JPG the camera is doing all of the processing that you would normally do to a raw (NEF) file. If you shoot NEF then YOU have to do the work. But the trade off is that you get a better job done. Different raw converters make different starting point assumptions.