great post.I couldn't say what the lowest temp to trigger an RI would be, but with a hot spot of ~90F, I would say it is extremely unlikely that is the cause. I suspect his snake is just hugging the hot spot a little more than a snake with a higher ambient would. Now that said, an RI can result from a litany of other things, but unless the ambient temp is quite cold, I don't think it is often the case in cages with sufficient hot spots.
You can try the following:
(1) For troublesome snakes, the easiest method I have successfully employed is turning out the light during feeding. It seems to add security and makes them more likely to strike F/T mice.
(2) The enclosure you are using is extremely small, which in of itself may not be an issue, but from the picture it looks pretty open and light. What I mean by this is that the white surrounding self makes the area quite light and the coconut hide is not very deep. It is possible this is making your snake feel insecure. Irritatingly enough, some snakes just decide at a random point in time that this is a problem, even though they have fed under identical conditions in the past. I suspect there is some imperceptible difference, but often I am never able to figure it out. More useful is the solution - which almost always easily solved by cluttering the enclosure with paper towel balls, fake plants, or whatever. In your case you might try artificially darkening it to make it feel like a "deeper burrow".
(3) When I really have trouble, I keep my special gerbil helper, Scentmaster Bob, on standby. Turns out gerbils hate this, but rub the F/T prey item against the gerbil so it gets the scent on it, then feed it to your snake.![]()