I think it's great that you're asking these questions. You sound thoughtful and safety-conscious despite being pretty chill and able to adapt to the unexpected. These are good qualities for a diver to have.
The first time I lost an instabuddy was on the Avalon Harbor cleanup dive a year ago. I had a similar experience level to yours at the time, and I was in a trio with two more experienced divers. As soon as we started picking up trash, the visibility dropped to near-zero. It would clear up a bit and we'd find each other again, then we'd pick up something else and lose each other. Finally I lost them both for good. I searched for a minute, then surfaced, as I was taught to do. This is an impeccably organized event; the surface was lousy with kayakers and safety divers, so I might've even survived a heart attack right there and then. I let the nice folks know I was OK and then floated there another 5 minutes, waiting for my buddies to surface. When it became clear they weren't going to, I began heading slowly back toward shore, keeping my face in the water to look for them. When they eventually made it back, I learned they had lost each other at the same time they lost me, and they both just did their thing solo. One ended up way off the end of the pier we'd been sticking close to, probably farther than he was supposed to go, but hey, he found a lawn chair.
Several months later I did another cleanup dive in Newport Harbor, and was assigned a buddy who clearly communicated via his body language that he didn't want one. The viz was worse to begin with; he and I were less than 5 feet apart when we descended, and I lost him as soon as we dropped below the surface. I moved in his direction but still couldn't see him. I poked my head above the water and saw his bubbles coming up farther from me than he was before.
I had two options: abort the dive, or go solo. Option 1 would have been the responsible thing to do. I knew it at the time. But I chose option 2. The bottom was only about 15 feet deep. The place was again full of skin divers on kayaks ready to rescue us, and the harbor was closed to regular boat traffic. There was no surge, current, or other adverse conditions other than the low viz. I had gotten pretty good at navigating with a compass, had a lot more dives under my belt with less-than-stellar instabuddies with whom I felt alone, and had dealt with some minor emergencies. I didn't feel panicked at the thought of not having a buddy; in fact, I felt kind of relieved to be able to focus on myself and dive the way I wanted to. I survived my swim along the razor's edge and found lots of cool garbage.
The instabuddy horror stories keep piling up for me, and now that I have the requisite number of dives I'm looking seriously into a solo course. I'm also just not getting stressed out anymore if my buddy ditches me. If you stay close, cool; I'll do the same. If you take off without a backward glance, whatever; I was going to dive more conservatively with the uncertainty factor of an unknown buddy anyway. (With a good buddy, I'll go below 100 feet and surface with 500 psi. Without, I'll keep it shallower and at least double my reserve.)
But I still don't know what to tell people in your position, which used to be mine. You can't take a solo course until you've logged 100 dives, and if anything you need those skills more now than you will when you're eligible to learn them. You could take Advanced and Rescue, which would improve your skills and make you better prepared to handle anything that goes wrong. You could take Fundies and dive with GUE folks, who tend to take their buddy teams much more seriously. You could try to teach yourself some self-reliance by buying a solo diving manual online, getting yourself a pony bottle, and having an experienced buddy help you practice with it. But as long as you're diving without a familiar buddy, you're at risk of all the things that can happen to an unprepared solo diver, which is what you are. It's also difficult for you, at your present level of experience, to identify and accurately gauge the risks you're taking; you don't know what you don't know.
That said, I'm not going to condemn you. Ideally you should insist on a buddy who acts like one and refuse to dive with those who won't. Failing that, you should maybe stick to shallower, easier dives until you have a solid buddy and some more experience. But you did well sticking close to the guide, monitoring your depth and your air and surfacing (with a safety stop, even!) when you got low, and taking the time to reflect and seek input after the fact. I think these things-- common sense, self- and situational awareness, humility, and a desire to get it right, will ultimately serve you better than an equally inexperienced buddy even if s/he never leaves your side.
ETA: Wow this thread has moved fast while I was writing that novel. Yes, get a $200 entry-level wrist computer before your next dive, learn how to use it, and always keep an eye on your air and NDL at a minimum. Even if you have a guide and a great buddy, consider those things your responsibility alone.
