My aim is to reproduce my experience of Win7-snippingtool where, once its shortcut had been clicked, the next requirement was to select the area to be snipped. That is what I mean by 'one-click' snippingtool. In Win10, after clicking on the snippingtool shortcut, the snippingtool window opens and then you must click again on NEW before you can select the snipping area. That means two clicks before it is snip-ready.
If I run a batchfile at startup to launch snippingtool, then it is there open and waiting for NEW to be clicked when required; so that takes only ONE click, just like in Win7. However, since USING snippingtool also closes snippingtool, after that first use, I am back in the Win10 situation of having to click to launch snippintool, and then click to select NEW, before it is snip ready again - I am back to having to click TWICE before it is snip-ready.
Keyboard shortcuts to my mind simply shift the tasks from the mouse to the keyboard - I don't want to change the type of action, but reduce the number of actions.
'if I need it later I just leave it open.' Thar is the crux of the matter IN MY CASE - I can NOT leave it open - it closes after use. My Win10Pro is ver 1151 - maybe your Win10 is different with a different snippingtool behaviour.
Lastly, it seems to me if I run win7's snippingtool in win10, but in compatibility mode, that should restore the behavior to that in win7, but FOR ME that tool can not run in Win10.