"•If a knife enters just at the bottom of the ribcage, angled up towards the heart, how likely is it that the injury is entirely non-fatal?
•I need the character to stay conscious for five to ten minutes after getting stabbed; is that probable/plausible?
•It would take her friends about twenty to thirty minutes to get her any proper medical attention. How long would it be before she'd wake up in hospital, what kind of physical state would she be in then, and how long would it be before she's up, about and in reasonably good health again?"
She'd have a very high chance of dying, but not guaranteed. It could miss her heart/major blood vessels, but she'd have to be extremely lucky. (I heard of a real-life 3 year old who survived having her throat slit to the point of her windpipe being severed, simply because her head fell in the right way to close the wound. This is more plausible-sounding than that was.)
Even if she died, she'd have a pretty good chance of surviving & staying conscious for a couple of minutes. Heart injury isn't an instant death, the way head injury can be. For example, when my father used to hunt deer, he said a head shot was instant death, but with a chest shot they might take a couple of steps.
She'd probably be heavily sedated in-hospital to lower her metabolic requirements and control pain. Plus, she'd have a risk of brain injury from lack of blood flow/oxygen, which might result in coma. Ballpark might be somewhere from several hours to a day or two, more if she had a serious brain injury.
Recuperation would likely take months. She'd spend a fairly short amount of time in acute care, then be transferred to longer-term care in a different hospital ward (or maybe another hospital) and then finally go home and be taking it easy for a long time.
If I'm asking for advice on a story idea, don't tell me it can't be done.