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.
Your players are invested in a different game.
The first group that I played with played the way you describe: mucking around town, not caring much about objectives, messing with every NPC they meet. It took us entire sessions to get through even the most basic of quest-giver scenes, and we delighted in spiking plot hooks and keeping the DM on his toes. I still play with that group, and it's still a lot of fun!
It is likely that your players have a very different view of the gaming experience--maybe socializing or being "off-task" is just a form of relaxation. While you may want to push through the prewritten campaign, maybe they want to just blow off steam and mess around in a fantasy world, doing things they'd never do in real life. Both viewpoints are perfectly valid ways of approaching a TTRPG.
On the other hand, the group I DM for is very plot-oriented---they will eat up quest hooks, and they will efficiently pursue their goals. When I first DMed for them, I was surprised that they completed a quest in the time it took my other group to stroll through town.
Adapt or pass the baton
Trying to force those players into "focusing" on the game will likely just swap your roles: you might be satisfied, but your players might chafe against your "railroading". The first DM of my first group tried to do this, and it caused a lot of friction in the group. Eventually, that DM became a player, and we had a different DM. This DM, instead of trying to run a coherent plot, simply had an omnipotent, insane NPC wizard teleport us around and told us to cause chaos. As you might imagine, this went over a lot better.
Additionally, instead of making deep dungeons or storylines, he focused on making more interesting NPCs and environments instead. This NPC focus meant that he was prepared to create interesting scenarios when we were messing around in town, and he didn't waste time on dungeons we would never enter. For example, in your tavern, you can have a champion or something challenge the PCs to a 1:1 fight, or give the shopkeep a interesting prank magic item (horn of baubles, maybe?). If your players like wandering off or doing individual things, plan more individual encounters or ones that don't require the whole party to be present. I realize you're trying to run a published adventure, but maybe that's not suitable for this group. You will probably get more mileage out of the adventure by using its encounters and NPCs as inspirations for your own modified campaign, rather than running it straight.
Basically, the three questions you ask at the end are basically, "how can I force my players to play how I want?" Instead, you should be asking, "how can I adapt my campaign for these players?" If the answer to that latter question makes you not want to be a DM anymore, then you should pass DMing responsibilities to someone else.
Best Answer
Do what authors do when dealing with this kind of issue. Just have some quick exposition and move on.
If I have a party that is in one village and they need to get to another village a long way away but I don't want to have anything important happen along the way, then I just describe the journey and have them arrive at their destination. This doesn't mean that nothing happened along the way. It means that nothing important to the story happened along the way. The thing to remember here is that the 1500 miles is only important if you and your group want it to be important.
To make it seem a bit more important without having to actually play through a crap ton of random encounters, give the group a few XP and then ask each player to briefly describe one encounter they had along the way. Perhaps pushing a little for specific people they met or places they stopped. This puts some faces and personalities along the route so that if they ever travel that way again and you need some repeat characters, you'll have some.