I wonder what it is about your moms that was successful. Was he just more tired overall, or was it just easier to get him down? How is it different, ie the sounds he hears falling asleep, the texture of the blankets/sheets (anything), the type of bed (the same one?), is it darker/lighter in there?, the amt/kinds of activity he is doing before going to sleep successfully, smells?, clothes he is wearing; anything that stands out?
Your comment about your lo being outside the box for a lot of things like feeding (oral defensiveness), sleeping (if there is difficulty screening sensory input in the environment it is hard to sleep), or being in louder/new environments (new people, things, toys, noise, smells; how do you decide what to pay attn to when it is all new or overwhelming at once). It does take an ot with the right training and experience with birth to three, perhaps. Although, I know you have someone who would be pretty well qualified.
The ot we have met with told me "You know you have been in this business a long time when you are telling parents which fast food restaurants have the biggest straws for working on blowing and sucking activities. She wasn't advocating us eating there with his food issues, but rather it is the only place she has seen them available. (He was 2.25 yrs before getting how to use a straw; an issue with low tone.) She was telling us that Burger King had the best straws, so we can work on the low tone H has in his face. It is probably our biggest sensory challenge in addition to some lingering feeding issues that the ot says we still have from reflux. His proprioception is doing well (an initial concern); he is riding a run bike!
OK, back to discussing sleep, lol. XXXXXXXXXXXXXXXX, I hope you are having a good day and you find some solutions very soon.