Yes, yes and yes. SimplyPrint doesn't restrict you or your printer in any way. So if you still occasionally start a print through OctoPrint, Mainsail, Fluidd or from the SD card slot, you can still do so - and SimplyPrint will even still monitor your print, and show you the progress.
You can also still keep using whichever slicer you prefer - you're not forced to use the SimplyPrint slicer (even though you'll be missing out! ;))
This also means, even if SimplyPrint goes down (which is very unlikely, we have a 99.99% uptime), you will still be able to interact with your printer in the old fashioned way.