My computer has issues so I can’t click on your images, but to address some of the things you mentioned in your text:
-You wanted to know about binding to shift + button. Assuming you are using an action mapping, the correct way to do this is in project settings inputs. You can simply check a box to let it know that it requires shift as well as the listed button. 
-If your launching is inconsistent, you may not be overriding the previous velocity. The override check boxes mean that it sets everything to 0 first and then proceeds to launch your character with the exact velocity you input. If you don’t check the override boxes, then your current velocity at the time you launch your character will be taken into account (and may counteract part of it).