I'm not sure you found an answer, so here is mine: mobiledevice column will stay undefined unless you match it with mobile_id column, with a specific lookup table (can't remember the name). It works like browser.tsv lookup table, except you have to ask Adobe Customer Care to get the file.
You should ask to get the file periodically since new devices appear regularly, with a new mobile_id to match with its name. Usually you get a new file every month.
In place of relying on USER_AGENT sent by browser I would custom set platform and device type ("desktop", "mweb", "ios", "android" etc.) value in a custom propX and than look for post_propX in data feed. If you want to go a hard way to find it... Following links can help you find a way out.