How should I go about choosing an adventure for a group of new players?
The most important thing is to find something that speaks to you. If the premise leaves you flat, you are not so likely to be able to breathe life into the campaign.
The other most important thing is to find something your players will enjoy. Heck, they'll probably enjoy anything you DM cuz you'll be so great, but if they are all obsessed with dragons, maybe the module with the dragons, right?
What qualities should I look for?
Make sure the module you select is for new first level characters.
(Unless you really want to start at a higher level, which would make
your work a bit harder.)
Modules labeled introductory are good for starting DM's and players.
They often reprise the rules you will need to be familiar with, and
give points to where to look rules, etc.
CAVEAT: The word "introductory" doesn't have a fixed definition. Some introductory modules will be entirely self-contained, while others might require other materials, such as the Monster Manual and the DM Guide.
A module might be (A) specifically for particular game rules (like
D&D 5e), (B) be "compatible" with a set of game rules, or (C) it
might be just the "story" part without the monster stats. A module
of the first type will be easiest to use.
...and how can I tell if a published adventure has them?
I think that a trip to your Friendly Neighborhood Game Store might serve you well. The staff of typically pretty knowledgeable, and you can flip through the merchandise. Other customers often might offer their insights as well.
This site and others have chat forums where you can get suggestions and discuss.
You might also read product reviews online. For the in-depth info you are interested in, I'd favor review website like Escapist Magazine over reviews at online stores (although both have their place).
Should I restrict myself to official WotC adventures on the presumption that they have the best writing and playtesting or is there a way to identify third-party adventures with a similar (or higher) level of polish and quality?
Probably not a bad idea to favor the WotC's D&D 5E material, if that's what you're going to be playing. This has more to do with the encounters being tailor-made for 5E, than one company's material being better than another.
So which are those?
Applying my own answers to your criteria, The Lost Mine of Phandelver in the D&D Starter Kit, and Tyranny of Dragons both would be solid choices. Phandelver also would be a good fit for the number of play sessions you mention.
Tyranny of Dragons is an updated version of the early D&D 5e releases: Horde of the Dragon Queen and its sequel, Rise of Tiamat. Its updates include play balance fixes, perhaps most notably in earliest set of encounters, which make this module more appropriate for beginning DM’s and players.
NOTE: More recently, another official introductory module has been released: Dragon of Icespire Peak. I haven’t read through this one yet, so I cannot comment on its specifics.
You'd mentioned social encounters...
Most D&D modules have a whole lot of fighting involved, and the two I mentioned are no exceptions. Without doing any spoilers, Phandelver probably is more dense with opportunity for social encounters than Horde of the Dragon Queen.
The existence of opportunities for social encounters, of course, has a lot to do with your DM'ing. Monsters don't have to attack the party on sight, even if the module says they do.
Pre-generated characters save a lot of time, and remove the need to learn how to create a new character. Make sure everything they need to know is on the character sheet so they don't get distracted while reading the rulebook. D&D books are huge and complex. Another good point with that is that you can choose simple abilities that you know will be useful in your scenario.
Often when you play with the same group of people for a long time some habits of talking tend to replace official terms. When you are playing with beginners avoid the slang as it can be quickly very confusing.
Be helpful and forgiving: If a player move his character without noticing he would trigger an attack of opportunity when he could have avoided it, tell him that he can avoid this, show him how, and let him change his action if he wants to.
Considering rule simplification be careful you keep the game interesting enough for your experienced players. Maybe you can give the new player a free special ability which removes the need for magical components (or simplify an other part of the game). It could make you able to simplify even more the rules for the new players.
Best Answer
The power to wield spells comes with a little more investment into the game
If he wants to understand his options and make informed choices himself, he must slog through the new and unfamiliar spells as extra study. Let him borrow your PHB or have him read through online spell lists on his free time, to familiarize himself.
What you can do to help him is turn the spell selection into an away-from-table activity (or better yet the whole level up process) - homework, if you will. This ensures that the game doesn't grind to a halt as everyone picks spells, rolls for additional HP, and reads through new class features.
With this method, you may even communicate with him through text, email or other media for any questions he may have regarding the spell, but most important of all: It doesn't consume precious game time.