I’m sure you will get lots of advice, but o answer your first question. Eurostar is super easy to use, easier than an airport and flying, although not too dissimilar security wise.
However, do not stay near or hang around Gare Du Nord. Get straight on the metro or in a cab/Uber and head to where you’re staying. Also depends where you’re coming from in the UK if you really need to go from St Pancras, as some of the smaller stations are perfectly convenient.
Do not expect Emily in Paris vibes. It’s a gritty city. Paris can be quite heavy on pick pockets/begging in comparison to UK cities so be aware. If you aren’t city people here, practice the blank stare to ignore or a firm Non!
Personally, I prefer hotels to Air BnB, find them more convenient, but think about your eating habits.
Regarding areas, do some research on what you want to see. You can create your own google map for your trip, which you can then share with your friend. Plot on the map the things you most want to see, you may find an area is best in that most things are walkable, although you will obviously have to travel out of town to Versailles.
Very touristy, but I love a boat down the Seine. Marais and Montmartre, passing La Maison Rose are classic Paris. Opera Garnier is stunning, so is La Grand Mosquée and La Philharmonie.
I tend to make city breaks shorter, you will be on the go a lot and would find 7 days too much. If you do the map you can probably plan a rough itinerary and see how much time you think you would want there.
Don’t give yourself too much to do to the point that you can’t wander, relax and people watch. Charging from planned activity to the next can be intense and doesn’t allow you to soak up the city.
Definitely plan a few restaurants or cafes you would like to visit, can help if you don’t find what anywhere else.
I think it’s good manners to learn a few words of wherever you’re going, but if in doubt a picture (on your phone) helps.