I'll start out by saying that I am not a bunny expert, so take this all with a grain of salt. These are my thoughts...
Although the general advice says that animals need animal companions, every pet and situation is unique. It's true that a bunny left alone with no companionship whatsoever will be miserable like a human imprisoned in solitary confinement. I think that's the kind of situation the authors want to avoid.
But to say that it's impossible for a bunny to be happy without fellow bunnies is a pretty gross oversimplification. I'm no bunny expert, but I'm pretty sure there are tons of situations where it's better not to add another bunny to the mix, and that doesn't mean your bunny is destined for misery.
My single bunny free-ranges in my living room. We have an hour of snuggle time every morning. From 11-5 pm like clockwork she naps in the fireplace (!), and our family spends the evening together in the living room where she scampers around alternately lounging on the carpet, licking our feet, or sitting by her food bowl. At exactly 11:30 p.m. no matter what we're doing she scampers back into the fireplace and goes to sleep. I feel confident that she gets enough attention because frankly she's the one who decides when she's had enough cuddles--we're always game to give her more.
You're right that there are a lot of unknowns involved when you bring a new creature into your household. Bunny #2 will have their own needs to accommodate (maybe digging, chewing or spraying). And there's no assurance that it'll work out better for your bunny or for the other members of your household.
So I'd say bottom line is that you need to use your own best judgment about whether you can reasonably accommodate another bunny in your home, and in particular about whether or not your bunny is currently lonely --which may be difficult to determine, because humans tend to project their own feelings onto their pets.
I look forward to hearing what you decide to do!
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