I never finished a post I once started to write about "solo" diving that is consistent with this point. I think many divers are essentially diving solo even though they are diving with a buddy. If your buddy is too far away to reach them or get their attention, they can't really be of much assistance in an emergency situation unless they happen to turn and look at the right time and see the problem. In a drift dive situation, a buddy leading in a single file arrangement might not even be able to get back to a diver in trouble behind them if the current is swift and the distance great enough. I don't think enough time is spent in most OW classes regarding how to be an effective buddy once you start the dive. Executing some scenarios while in a controlled setting could sure help to drive that home.
With regards to the DI not checking the PSI of his student, I think he was exhibiting bad practice not just as an instructor, but as a dive buddy. I have hundreds of dives under my belt, my most frequent dive buddy has well over 10,000 and we both still check each others' psi at points during the dive. At first when I started diving with him I was annoyed at his practice of grabbing my SPG and checking it, but I now know it wasn't meant as a criticism of my dive skills, and this was particularly important when I first started diving as I was far less efficient than him on conserving gas during the dive and he used that information to help determine when to call the dive. Now that we are similar in gas consumption I will show him my SPG and he shows me his as we are nearing the end of the dive. I think it is just good practice to teach students this kind of practice as part of their OW training.
I don't think there is a good argument for grabbing someone else's SPG (not as a first resort anyway).
Far better to agree to ask their gas level -with two fingers together patted on the palm of the other hand twice. Response is as per prearranged signal - T(one hand vertical with the other hand forming the bat across the top of it) is common here for 100 bar with a single digit for each 10 bar over. So simple process - Diver A 2 fingered pat on palm, diver B responds T +4 fingers = 140bar. T+ 8 digits (either 5 on one hand and 3 on the other or 5 and then 3 on the same hand)=180bar and so on.
Now if the refuse to acknowledge the signal or give a response, the buddy still needs to know and would ask to see the SPG at that point.