Some of your fears are founded, others not. Fortunately there are mods to deal with everything, I'll just name them, and if you are interested will track down the download links.
"MoonlightGraham;d-996190" wrote:
In the past, I've only tried this for very brief periods of time, long enough to allow grandparents to establish a relationship with their grandkids who live in another household or set an unplayed Sim up with a career and some skills. When I do this, I return to my "main character" and find him doing relatively innocuous things. Browse the Web is a common choice, and I'm fine with that.
This is called loading screen autonomy. Two different modders have changed it so that the sims do not do those random things, but stand outside the house instead. This can cause minor issues at the active career lots (criminals at the police station will be outside their cells), but it's one of my must have mods regardless.
"MoonlightGraham;d-996190" wrote:
But if I spend several hours (in game time) playing another household, what might my "main" Sim start doing to fill the time?
Will he go to work if he has a job? (I know the Sims I've created and added to my world skip work constantly. They're always home during their work hours, or working out, or hanging out at a café, but it doesn't seem to hurt their careers.)
Careers are frozen when a household isn't played. The only problem occurs when you return to that household while your sim should be at work. You can send them in late, but they will suffer a small hit from being late.
Sims that do not have a job might get one while they're unplayed, at a random career level. But sims you leave while they already have a career won't change that.
"MoonlightGraham;d-996190" wrote:
Will he wander the Simworld, gobbling up food everywhere he can find it, including wasting Simoleons at food stalls when he's already full? (I read somewhere on the forum that this was a thing.)
I'm not 100% sure, but I think no money gets deducted when your sim autonomously tips musicians and buys food. Not sure about retail and restaurants either.
In any case, sims will gain weight over time from eating at food stalls.
MCCC can freeze physique altogether and FitnessControls can make it so that only the sims in your active household gain and lose weight/muscle.
"MoonlightGraham;d-996190" wrote:
Will he establish friendships and romances on his own, hitting on married Sims and causing drama?
Yes, sadly. Whenever your sim meets another sim for the first time, the game checks if these two start at 50% friendship or romance. The romance part seems to favour opposite sex sims, what can be a curse or a blessing, depending on your sim's orientation.
The mod to deal with this is NoRudeFlirtyIntroduction.
That aside, sims will start flirting indiscriminately with everyone when they turn flirty. Fortunately jealousy isn't as brutal anymore as it was in Sims2, sims won't outright attack eachr anymore, but turn sad first. From my experience it's also hard to hit on sims in an established relationship (but mileage may vary).
"MoonlightGraham;d-996190" wrote:
And, if I switch him to "unplayed" status before I take control of another household, will that stop him from doing lots of random things?
Don't ever do this! By taking away played status you give the game free reign to do with your sim whatever it wants, including changing their career. May I recommend MCCC? That mod allows you to fine control what unplayed and played but inactive households are allowed to do and what not.