Could we focus on this for awhile then...
Baha'is have to go beyond Krishna when they talk about a progression, because Krishna is not the originator of Hinduism. We have to go further back to get to anything that might be considered to be the "original" teachings of Hinduism.
I suppose we'd find a lot of "progressive" revelation or, I prefer calling it "evolving" of spiritual thought and beliefs. I think each people and culture kept evolving and changing their own beliefs to keep up with the times. Was it because God was sending new messengers, or, more like what I think happened, the people themselves keep creating their own ideas about what true spiritual reality must be.
CG when meditating this morning before reading this post, I had two distinct thoughts and they are both useful for what you have asked.
Firstly I do not know why you see that Baha'u'llah does not embrace more than Krishna in regards to the Hindu Faith. A Baha'i holds to the Biblical verse of looking for God {Good} in all things. There would be an inestimable amount of Messages given from God to the Hindu.
As discussed before, we can not know what those Messages were, if there is no accurate records. We know man passes down stories will always add their ideas to those stories.
The dreamtime stories in Australia are a very good example, some of the latest stories told by some current elders have been embellished by Christian and Materialistic thought.
I considered that all I can offer as a Baha'i is that each person of each Faith consider if all other Faiths have the same foundation in Spirit. In that way and in that way only, I see will unity be found.
No one needs to give up their faith, what they can do is accept the right of others to see it in a different way.
In that way we may consider the progression is actually growing in our own maturity.
The second thought that I had, that I may make an OP on, is the thought as to where God has chosen to give those Messages. They have all been given amongst people that had become very corrupt morally and materially, the result is, that the Message will have a great and lasting impact on those peoples and as it expands, other people.
Regards Tony