Hi @dvoinikat ,
Most of your queries are answered here in the official FAQ.
Adding little insights to give you more clarity for your questions
1. Portal Add-on and Page view is for "external users of Power Apps Portals. External users are those outside of your organization who sign in with a variety of identities such as LinkedIn, Microsoft Account, other commercial login providers, or anonymously."
2. The simple answer is Yes. If your internal users is going to touch the data coming from PowerApps Portal. Most likely it can be for administration activities or other sort of activities. Per App per Plan is a good start.
Tip: If you can estimate if the user is going to work on more than 3 PowerApps in future then you can go for "Per Apps Per User plan". This will avoid double billing and save you money.
Hope it helps.
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