There are shortcuts for Next Frame and Previous Frame (defaults: R and F)
The Auto Key feature is also helpful.
That should allow you to keep your mouse cursor on the Tree panel and not have to jump back and forth between the Tree and the Dopesheet. 50 frames should be less tedious this way.
Apart from those, no, you'll have to keep doing what you're doing right now.
I hope you understand Spine's system enough that it doesn't come out too hackish
(ie, you should put all the images in one slot instead of each in their own slot.)
There's a planned feature that's supposed to do what you're trying to do with image sequences, but with some extra features like repeat and pingpong and without you having to do things manually. But it's not in Spine yet and there's no word on when it's going in so I don't recommend waiting for it. It's not that things in Spine get implemented terribly slow, it's just that there are priorities and telling which features are coming next is a bit unpredictable.
