I only depend on other backpackers (or folks I meet) as "backup" in the sense that they could send help if I had an immobilizing injury.

To answer your "Big 3" questions: If I knew I wouldn't see anyone the entire trip, I might add a bit of quantity to my first aid kit (more ibuprofen, extra bandaids, that kind of thing), but I wouldn't change the basic contents. I might carry a spare day's food, just to tide me over if I couldn't travel for some reason (broken leg, snowed in, etc.) - and, if immobilized, the day's food would probably stretch to cover two days. Since my wife would notify the rangers if I weren't back on time (or so she says), and since I hike in the East, where the backpacking areas aren't vast, I figure I would be found fairly quickly. Also, the more remote the area or less familiar with it I am, the less likely I am to wander off-trail or change my hiking route mid-trip - no sense making it harder on SAR folks.

As far as the limit of expected rotten weather, I always take the worst forecast and add about 10%. As an example, when deciding about a winter trip, I take a 20 degree bag when the forecast is lows of 20, knowing that I'll also have my down pants, vest, and jacket I can wear inside the bag to extend the range another 10 - 20 degrees if the actual temps didn't read the forecast. On a spring trip, I'll often take that one extra piece (vest or longjohn top) that seems totally unnecessary. Another example: last weekend, the forecast was 30% chance of isolated showers. I simply assumed that we'd be getting a good rainstorm at least one afternoon on into evening. So, even though I don't normally bring a footprint, I brought one this time so I could pitch the footprint and fly first, then put up my tent from the inside, keeping it drier. (Raingear is a given on all hikes around here.)

But none of this is any different than what I normally do - it's just SOP.

I've never had to borrow from another hiker (OK, I don't count the idiot situation this last weekend where my gallon ziploc trash bag went AWOL from my pack somewhere between the house and the trailhead, and I shared a buddy's. I could have just kept my trash loose in my food bag, and washed the bag when I got home, but sharing with him, and carrying the bag the last half of the trip, was more convenient - and he offered, I didn't ask.)