There's not really any *content* missed by not being Scarlett's girlfriend, it mostly just changes dialogue. The only things are during her 5th scene, she brings you flowers as it's more of a "proper" date and during the day 60 sexy party, it's currently only possible to make her cum during her show if she's your girlfriend, although that will change with the next update.
You will have another opportunity to be Scarlett's girlfriend at the end of her next scene, but the same caution will apply (although I'm not fully settled on exactly how severe it will be).
As for April, as long as you've run into her in the library, she has a chance of appearing in the cafe at midday on any day, as long as you've kissed someone and haven't had a random event that day. I suggest turning the morning event probability down to 0 so it doesn't take your event, go to a new day, wait til midday and then just go in and out of the cafe. If you get a different event, go to the next day and try again.
Also worth double checking you haven't already had the scene and just forgotten (it happens a lot!). Check the replay gallery, and the message system to see if you can text her.
Good luck!