First off, the telemetry antenna is blocked by all the wiring near it. But the plane will still fly even with no telemetry data making it back to the Txer. However, if there are spurious emissions going on in the adjacent wiring, the telemetry antenna can also pick that up causing other RF issues.
The two remotes that are out in the clear look OK and those are probably giving you an appropriate # of fades. You did not tell us the ID letters (in the Pics) for the various remotes. So it is impossible for us to figure out which one has what # of fades. But I have a couple remotes just like that in my Sprint. However, they have the old antenna design.
Lately, it is NOT unusual to see 400-800 fades in a 6 minute flight. The one on the tank is probably not working well. We typically do not put them on the tanks. However unless there is Kevlar or carbon fiber embedded in the tank this location is typically not a problem. But, as previous stated, it should not be pointing at the Txer. But on the top of the tank is a great place to put a mount and get the remote up in the clear under the canopy. I probably would not have it horizontally polarized as you already have that covered with one of the other remotes. I would get it at a 45 degree angle up and not have the end pointing at the side.
I am wondering if you may be flying to far out or if the transmitter is operating correctly. Or if you have the transmitter's antenna pointing directly at the plane, most of its signal is lost into the ground or not radiating toward the plane. Typically, these potential issues are not a problem, but can be. For me, a frame loss tells me something is not right (i.e. all Rxers missed a frame) Others accept several (e.g. < 10) frame losses because a frame is only a 22 ms signal disruption and there are a lot of frames sent in a second. So basically if you have under 800 or so fades you are just "fine tuning." I tend to dabble with fine tuning the remotes because I have been working with RF devices for over 50 years and I am not used to seeing frame losses.
On another note, I have noticed that since the remotes now only have one antenna, I have been seeing more frame losses when we troubleshoot at our events. It used to be I very seldom saw frame losses, once we had the remotes in the best places. Now, ... it seems like I am seeing more frame losses, even though it looks like the remotes are positioned OK. From an RF perspective, it makes sense. Going from a dipole arrangement to a quarter wave may still work. But, going from a dipole to a quarter wave antenna reduces antenna gain. Also, a quarter wave is typically more sensitive to its surrounding environment. For me, the jury is still out on the current remote design. The current design may still work, But is an optimal solution? There is only so much you can do with the electronics to compensate for a less than optimum antenna.
Good Luck. Have fun with the Sprint. After several years and hundreds of flights, I still love mine.