Hi guys,
Firstly:
I just wonder: how the transmission work between Spark - Remote controller (RC) - Phone?
My guess is that:
-Spark connect to RC, then send the signal to RC.
-Then RC connect to phone and transfer the (image) signal to phone.
Is that how it works?
If that is the case, considering OTG got no affect on Spark - RC connection; and RC - Phone are perfectly closed to each other for the wifi.
--> Why OTG make the better video feed and range?

Secondly:
DJI said OTG is not recommended, then if something happens to Spark (Spark sudden power off for example), will it still be cover under warranty?

As for the crash, data analysis will be helpful to check the exact reason. And corresponding solutions will be offered based on the analysis result.

I would say that, reading from Spark manual, the usb on rc is for rc charging only. It has never been intended to use as mobile device connecting port. The firmware programmers might forget to disable the link and that make it becomes popular among trouble users.
Spark rc is designed to connect to mobile device via wifi only. With that very short distance between rc and mobile device installed on its holder it is almost no interference between them. So, the video feed and connectivity problem can be caused by the use of improper mobile device, especially those android phone and tablet that have not been tested by Dji and the interference of the flying environment. Even you use the usb connection, which may not be done for so long (Dji will remove it soon), you can still encounter the same problem if you run Dji Go 4 app on non-tested device or flying in inappropriate environment. Scan this forum well.

One more thing. Dji Goggles does not support Spark for now. It is designed specially for Mavic. But you can connect it via HDMI to mobile device like iPhone or iPad by the use of Apple's cable.