I don't really see a problem, I mean there are mmos out there where you can add certain npcs to your party, not to mention that Shiroe stretched and emphasized that this isn't the Elder Tales that they knew and that they're not npcs. Also I don't think I've ever played an mmo where you have to be friends in order to add someone to the party.

As for Rudy knowing own skills, how is that weird? The other mage pulled out a ton of windows when explaining the different kinds of magic etc to Shiroe so it seems reasonable that other people of the land can do the same. It's also the most logical conclusion if you think about food, they've stated that food all taste the same when cooked through the menu and we know from their trip to the castle in Eastel when they talked about their chefs that they've started cooking without the menu since it taste differently. Marie talked about the people of the land also wanting to eat good food, so someone has to cook food for the people of the land before all of this, so how did all of them cook before when food all tasted the same if not through the menu?

As for the telepathy in the latest episode my guess is that they're talking through party chat, if Rudy is already in the party he should automatically get connected. After all Minori talks to most of her party and since we don't see her calling each and every member one by one when she has something to say the logical explanation would be a party chat, it'd be very impractical otherwise.

As for levels, monsters have levels why can't npcs? I don't know if I'm just making stuff up now but for some reason I remember them talking about how npcs can't level up to be as strong as adventures so that their level cap is lower than adventures, take it with a big grain of salt as I can't remember where they said it and I don't feel like rewatching every episode right now.