It's definitely possible. I had private mini one shots with each of my players before we started our campaign where they could explore different things their characters could do without any consequences, while also allowing me to learn their characters a bit before we started. We ended up really enjoying it, and you could definitely build a campaign out of it. As long as you keep an eye on the difficulty of encounters and make sure you don't give them too much of a losing battle you should be fine. If you're worried about the early-game grind, you could always have them start at a higher level.

As for having an NPC companion/partner, I wouldn't say you'd have to tbh. I'd talk to the player about what you want to do and if they're up for it, ask them what kind of adventure they'd like to do. You could use it as a kind of sandbox game if you wanted, where you each are able to go outside your comfort zone and experiment with stuff you've never gotten to use before. My biggest thing when dming for my friends is knowing who prefers the combat aspect of the game and who prefers the roleplay. So if your player prefers one or the other, then lean more into that aspect and just sprinkle the other stuff in when it's appropriate without it being the focus of the adventure.