Photoshop :: How To Non-destructively Sharpen / Resize And Save Images

Ok... here is my dilemma. I am having an incredibly difficult time understanding the best way to sharpen, re-size and save my images for both posting on the web and giving them to clients. I completed my first paid photo shoot (yay!), but as I finished editing each image, I re-sized it and posted it on my FB photography page. I later learned from a fellow at my local print shop that this is a destructive and irreversible edit (not yay! ).
 
So...  before I pull out every last strand of hair on my head, I REAALLLYYYY need to get a good grasp on how to do the following things so that I can establish a good workflow: 1. Sharpen my image well {w/ Smart Sharpen}. Does this have to be done on a flattened image... and isn't flattening irreversible?  2. Re-sizing my images for both web display and client work/printing. Is it true that once I set it to 72ppi for web display, that I lose a great deal of the detail and quality? Do I need to create a copy of the file and have 2 different image sizes?

I am self taught, learning off the cuff through tutorials and constant error... and I just want so badly to have a smooth and beneficial work flow in place.
 
Currently, my workflow is as follows... 

1. Load images into LR and convert to DNG files 
2. Quick initial edit & then send into PS CS6 
3. Perform detailed/layered edit(s) 
4. {I know I'm supposed to sharpen now, as the last step, but am afraid to permanently flatten my image in case I want to tweak the layers later..} 
5. Save the file (unflattened) 
6. Go back into LR and Export the file to the appropriate place on my hard drive
 
So... at this point, my image is still at 300ppi {not appropriate for web display}, unflattened {I'm told flattened images are ideal for client work and printing} and not as sharp as I want it to be {because I don't know when to apply Smart Sharpen filter}.