Will the osmo hold an ipad?

Depends what size ipad, the osmo device holder will open up to approx 170mm
See this YT video, adaptors for larger devices

No, it will only hold cellphones.  Unless you're talking about 3rd party accessories as in #2

Wouldn't that be a bit heavy? I'm finding that by the time I have my iPhone 6s Plus and NTG2 microphone mounted the whole setup is a bit heavy for extended use.

Portrait mode would be even better, but it requires the ipad app to allow portraitmode.
Physically it fits and it feels balanced and provide a comfortable place to balance the camera....with just the phone it is a little side heavy.

On the Android nexus5x with nougat the DJI Go app allows for portrait mode, as does the iphone7 version....why can't the ipad version?
The extra screen would be helpfull for larger image and space for histogram etc