1: It's unlikely but he probably knows a good amount about how the universe works and how the different planes and realities interact with one another.
2: Bard's true form is basically stardust and light, so by saying "We haven't seen his true form" it's essentially saying we haven't seen him without his garments, which won't really happen because the nature of Bard's existence means he's practically invisible without clothes on.
3: There's no connection between Bard and Ao Shin's releases. Ao Shin hasn't got a kit yet, that's why he hasn't been released.
4: Champions and beings probably exist that understand Bard perfectly fine, but that doesn't mean we won't see more in the future. I'd recon Soraka can understand him alright, but something to keep in mind is that Bard's form of communication isn't completely unintelligeble' while the words don't make sense at a glance you'd garner the meaning if he was speaking directly too you.