Even if he wasn't kidnapped they would've spent time looking for him regardless, so I think searching for Yuan was always intended. However, the fact that much of the lore uncovered in Bailu appears to have been part of the original plan leads me to think the kidnapping probably was, too.
Searching for Yuan is the perfect reason for Ryo to actually spend time with Shenhua, and build a bond between the two by doing something that benefits both of them. I mean, you can't just say two characters' destinies are intertwined and then not actually have a good reason for them to spend time together.
I guess you could argue the plan could've been for them to search for Yuan, only to find that he'd been murdered -- and now they're both out for revenge against Lan Di! -- but that doesn't work at all for me.
Firstly, you already have the "father murdered" parallel with Lan Di, where it carries a lot more weight. Secondly, Shenhua is the yin to Ryo's yang. She is calm and Ryo is hot-headed. Murdering Yuan at this point in the story, before we've even gotten to know Shenhua properly, would be a sloppy move from a narrative perspective. (Not saying she won't be tempted by a form of revenge at some point, but not now).