Question: I have Obsidian Edition on Steam. But I want my iOS to be the main save. If I link my accounts will the Obsidian Edition bonus content show up on my iOS account? Will I lose any of my purchases on Steam if I merge it into my iOS save?
Yes, I had a similar experience. Main characters on iOS, then purchased the Steam version with Obsidian perks. Post-merge, your account will be the sum of all your content (iOS purchases, treasure cards, gold + Steam DLCs, treasures, gold).
Parties and characters don't quite stack together in an "additive" manner, so the platform that generates the key gets to be the "master account" for the purpose of the merge.
Generate the merge code on your iOS device, and then enter it into your Steam platform. Once the merge completes (might take a few minutes), your iOS PFID and Saves become linked to both your iOS and Steam platforms, and the total content you own on both platforms is available everywhere. After a restart, both platforms should have identical content.
I don't know if the merge has changed since launch to address what happens when you have different characters on both platforms, but it is pretty safe to assume that should there be any conflict, the device that generates the key (iOS in your case) wins out.
You shouldn't lose any content in the process. Just take care which device generates the link key. That PFID is the one that "survives" the merge process, and has all the DLC, gold, treasure cards, etc... of the other merged into it.