There are no ‘downstream effects’ to understand for them. This is an internal API used by Chamberlain for their own products. It is not intended to be used by any third party and there is absolutely no obligation for them to maintain it for third parties who reverse engineered it, nor to ‘clarify’ anything to anyone at all. This kind of entitled behavior is not going to help at all. In fact, worst case, it could push them to pull a Mazda on the current integration.
If you really want them to open their API and document it (and that would be great), then you have to lay out the advantages this would afford them as a business, financially and for customer retention. Opening and publicly maintaining an API is a lot of work and costs money. There must be some form of ROI in it for them. If you can clearly show that, they may listen to you. This is not going to be easy at all.
The best approach, obviously, is to drop the cloud API altogether and go fully local.