It is actually a pretty easy fix in any supposedly "New Urban" development. All of the carriage houses are supposed to serve as the flexible part of one's living space - boarders, family members, post-college kids who can't afford their own home, live-in staff, you name it.
Of course, it works better if you weren't completely overbuilding your development so the carriage house and main house had to have the same utility meters so they counted as 1. ;-)
And I've shared a house (and bathroom) with complete strangers before. House's owner had the master suite w/ her own bath, another renter and I each had our own bedrooms and shared the hall bath, and everyone shared the kitchen etc.
When you don't have anything worth stealing, respect people's spaces, rent to people you know, or are all the same gender, most of security issues you mentioned cease to exist.
Hmmm... Somehow when I took a stroll down the following little old lady's path, the solutions did not feel as easy, New Urban tidy, or youthfully possible:
Imagine... You're a 75 year old childless widow and everybody you
know and trust is either dead or dying. You "came up in a past time of sanity, common honest goodness & shy stay-at-home housewives". Everything & everybody in today's world feels scary & unfathomable to you. The sentiment of "your cherished home and things" are the last vestiges of comfort amidst chaos. Your dear departed husband of 57years, made some seemingly wise investments to insure your golden years comfort (in a sunny retirement community 1200 miles from all you ever knew); unfortunately the investments didn't pan out to be the safe/sure return that "nice young" investment counselor had assured couldn't fail. Hubby had a fatal coronary, and left you holding a bag you're unequipped, financially or emotionally, to deal with. Your
modest manufactured ranch style 3 bedroom retirement home (drat! nary a carriage house in sight) is at risk because the failed investments were funded with a sizeable 2nd mortgage against a temporarily inflated home equity. Your chronic health ailments are an obstacle to securing any sort of income producing job.
How are you going to survive and hold onto the things you
hold dear, worthless though they may be (in case the "nice young" GIRL who answered your ad that you're considering renting a bedroom to turns out to be a crazed meth addict who steals your mamma's china and your old portable TV to pawn for her next fix)? You're just too old, too tired, too sad, and too afraid to even begin to know where to turn.
Maybe our local Habitat for Humanity could stop building complete houses and transform their "business model" into one of adaptive remodels for deserving little old ladies like this one?
Maybe vo-tech or high school shop class projects could include such ideas?
I've been living that same philosophy too, about 450 miles, but now that i have little ones, I wish i was home. 
