OGL Shett with Custom Roll Template?

Is there a way to create a custom roll template and use it while still using the 5e OGL character sheet?  It feels incredibly bizarre that the ability to create custom roll templates depends on giving up the 5e OGL character sheet (which means also giving up Charactermancer), but that is what everything I have read appears to suggest. I don't want a new character sheet or new NPC sheets, I just want to be able to create some nice looking roll templates for various actions in the chat window.

You've got a few options by default. You can always opt to use a custom version of the sheet (IE: You copy the code, and edit the roll templates how y ou want, then put the code into the "custom" option on the character sheet.), pros also have the option of using an API script to put custom sheet templates into their games (I personally use the Power Cards API)

My DM has a pro account, but I am the technically savvy one of the group.  Is there a way for me to utilize the API in games where the DM has a pro account, or do I need a pro account myself in order to be able to create the scripts for him?

As long as the creator has a pro account they can add the API from the settings tab of the game details page. The DM needs to add the script to the game. After that, anyone in the game can make use of the script. 

Really, what you can do is also just copy the sheet code from Github and add some extra custom roll template code at the end. This part is completely straightforward, since it's just an addition; it also means that it's easy to keep up to date with changes to the OGL sheet, since you just have to replace the copy&pasted part. If you want the buttons on the sheet to use  those new templates, it gets a bit more involved. Using the API & Power Cards may be easier than creating a new roll template from scratch, though.

For peace of mind, I would suggest going the Power Cards route. Not only is it infinitely more flexible (since it can be customized on-the-fly), editing the OGL sheet means that you would need to manually upgrade every time there is a new version published.