It's very common to exercise bettas like that. I have girls so I've never done it, but there's nothing wrong with it. It depends on the Betta. Some of them love it, it stressed out others. Depends on your guy. Don't worry.
I've kept a giant colony outside. Was incredible for a few weeks until the water bugs found it and bred. They are all the daphnia. Before that I could get hundreds with a single scoop. I definitely recommend trying it.
Your bettas wellbeing should be more important. If shrimp end up not working out, try something else. I don't think it's worth keeping a Betta in such, frankly, inhumane conditions. If you wanted shrimp, you should have waited before getting a Betta. Now that you have one, you have to be...
Time will tell. You'll definitely notice if it gets worse. The fins will curl and waste away. You can start treatment now or keep monitoring him and see if things improve. He's flaring which is good because it means he's acting healthy. Your choice when to medicate.
Nice! Most people recommend you continue to treat for 1-2 weeks after symptoms are gone. The parasite could still be in the water after all. I'd treat at a lower dose of 20-50% for at least a week, just to be safe.