For the first, you can re-select the same entity and that will deselect that.
For second, after you've placed the second point double right click and select "select" Or click on the line command in the sketch toolbar Or press L (shortcut for starting or ending the line command).
Brendan, Welcome to the learning curve of SW, the curve is exponential, so you will always be 'getting started' (fighting) with something new.
1) my habit has been to ESC and reselect the tool, a better option (might be) is to keep arc drawn and delete/change relations accordingly
2) when sketching lines, two options and different results;
a) LMB (left mouse button) select starting point > release LMB > move mouse to intended end point > LMB - new line continues from end point
b) LMB select > hold LMB > move to intended point > release LMB - line will be heighted/selected to added relations
my habit is to always hold LMB as I move, so I do not continue to draw lines
SolidWorks gets more intuitive as you use it. Best of luck,
The ESC button is almost my favorite.
Right click (RMB) is my friend...