Subaru Warranty Issues with Transmission Replacement

I'm hoping to get some advice and help on a problem I'm faced with Subaru. Last week my car broke down after I drove it about 2 1/2 hours on the freeway, and it turns out my transmission was fried. I had taken it to a Subaru dealership the week before due to a humming noise I'd be hearing when I accelerated 35-65 mph. The person at the dealership who went on a test drive with me said it was probably a wheel bearing and that I should have it checked, yet it wasn't an urgent issue. I explained I'd be taking a road trip 6 hours away and he said that was fine.
The following week my oil light came up so I got an oil change at an Auto shop the day before my trip. The auto shop did not check my transmission fluid and say that they did not touch anything with my transmission. In March however, I had my transmission serviced at my 30,000 mile check.

The problem is that Subaru service person is saying the cost isn't covered under warranty because there was no leak. They say that they found there is no transmission fluid, and are faulting the auto shop. What's also strange is that no light ever came on to indicate issues. In my reading about the issue, it looks like I should have had a warning light to let me know there was a problem. Also, the dealership should have checked my car the week before given the humming noise likely indicated a transmission issue (unbeknownst to me).

Help/guidance on how to navigate who pays for this is welcome!!