2) Model gliders don't seem very comparable, launching fast and then gliding at RN's significantly lower than stunt stabilizers (which usually get different treatment from wings). BUT...I'll think more on and look further into the CL's vs. RN's and tip actions. You have me going back to basics on this.
I was just thinking of the gliding part, but, depending on the glider event, yes, you'd need to optimize over the whole flight. I thought that Lee Hines's Sweepette had a peculiar planform until Gary James showed me why that swept ellipse works. I wouldn't think the aerodynamically optimal wingtip shape would vary much with Reynolds number, nor would I think that lateral-directional stability derivatives would either, considering the mechanisms that affect them, but holler if you find data. I don't think looking for it would be too fruitful. What would be is the variation of lateral-directional stability derivatives with different wingtip shapes. All I've seen old NACA data for is taper and sweep.
I'd hate to think that CL stunt is the only aero endeavor where some of these forms make no difference.
Randy Powell has convinced himself that stunt wings with lots more aspect ratio than usual are not the bees' knees. Stunt planes have settled on some moderate aspect ratio. Were I to do a trade study -- an important concept for anybody "designing" an airplane or part thereof-- on wingtip shape, I would probably hold effective aspect ratio constant and try to figure out the consequences of the differences between candidate configurations. Below is a picture of an Impact wing compared to a wing with the same effective aspect ratio, but with Flite Streak wingtips. It's approximate, but gives the notion. The red wing is supposed to have the same area, but with less span. Differences that stick out to me are sweep affecting rolling moment due to sideslip, leadout restoring moment for gust upsets, aerodynamic center, and propensity to twist in maneuvers.
3) Maybe, maybe not. What I'd probably do is explain.
I've tried. Do a search here on "rolling moment due to sideslip".
This looks kinda obvious to me, but stuff that one spends a lot of time on seems obvious after awhile, and it becomes hard to see why others don't get it immediately. I have flown combat for 50 years. about 40 years ago I started wondering if there was something I could do to avoid "hinging", as stunt fliers call it, in downwind maneuvers and having the airplane blow in at me in upwind maneuvers. I discovered tapered foam wings and NACA Report 1098. Aha. Then Gary James gave me a swept-forward wing that fixed the problem. Everybody else figured it out independently, and current combat planes can maneuver anywhere on the circle. The stunt flight envelope is different, but the effect is still there, and if I were to accept any more sweep in my wing, I'd want there to be a significant benefit to the trade.
The second thing that seems obvious is the notion of a trade study. I spent my last ten working years building and showing folks how to use a computer model to do airplane trade studies, so it's sorta in the front of my mind.
Wing configuration comes up on these fora every few weeks, especially wingtip shape. I spout out the same stuff. There have been so many repetitions lately that I've gotten even lazier about responding. Even without doing any calculation, looking up references to explain stuff gets kinda time-consuming. Nevertheless, I've taken three (I think) stabs at an explanation here.