Hey there,
Manager for a DT store in England here!

Coffee pairings should be done through personal involvement, never just pick something and go for it with the hope it'll work out. Thankfully you have an idea of what yo uwant to pair it with, now it's time to work on your Knowledge :) First of all, check here, https://passport.starbucks.com/desktop (ensure you are using your mobile phone to do this.. don't ask why honestly, just SBUX things)
This coffee E Passport will allow you to track different coffee's that you have tasted and allow you to also see what coffee's go well with certain items.

I am sadly not upto date with American based Coffee's, I know you guys have a little different to us. But if I had to guess, I would recommend either our solid Espresso Roast (Perhaps due to the Gluten Free items pick the Decaf version), failing that Cafe Verona (Aka the Coffee of Love) is another good option when it comes to cookies.
A final good recommendation would have to be either Ethiopa or Columbia Nariano. Show that you are trying these pairings in front of your manager, either on shift or in free time after you finish, and this will help to reinforce in your Manager that they have made the right choice.

The path of a Shift lead is easy, don't allow people to complicate it and tell you how it was hard or wasn't straight forward, those people haven't had the right training.
First of all, acting as you should. this is purely down to your knowledge and your ability to grasp and shift situations. Confidence is purely the key here, your partners will be looking towards you when an issue arrises to not just wait but to step up and own the moment right away.
Another thing to consider is how you treat your partners, remember even though you are ashift lead, you are also a barista first and foremost. Thus you must be level with your team and be on point, if you know someone is better on bar, get them on bar. Manage your team around what you are going to be doing, check your Year on Years, your Week on Week's. Plan your positions and change as is required. Merge in t oyour role as the Customer Support and help your staff manage a shift as best as possible.

If you have any specific questions that we may be able to assist, do not be afraid to send a message my way :) I've trained around 15 different team members from SSV to Barista's and am currently assisting with Manager training too.

Just remmeber, calm, smart and fair. That's the way to role.

Coffee pairings can be fun. Think of it as sharing your knowledge and love for coffee with others. I’ve done a few and really enjoyed it. I’ve also gone to a local reserve sbux, chosen a coffee and asked the barista there to pair it with something then sat and just enjoyed the experience for myself. Having that experience alone, I believe, helped me to be able to share it with my partners. I always look to the passport for ideas on pairings and look at the key notes in the coffees to make a decision on what I want to pair the coffee with. As part of my coffee master 10 feet, this became my project to get all of our baristas passports filled with notes on our coffees.

As for shift, I have no advice. I’m currently on the path to being developed for that myself but I just want to say congratulations and you’ve got this! Amazing accomplishment at 18 to be asked to do this role. I believe in you!