I've wanted to create a symlink from the Whatsapp media folder on internal storage to external. So I installed FolderMount from XDA, which is claimed to work. Instead, what I've seen is that Whatsapp is unable to write to the new location on the external SD card. When I click on a picture or other media file sent by someone, it downloads it but the blurred thumbnail persists, and when you click on it, it says 'the file was not found on SD Card' or similar. Does Lollipop deny writing to the external card for apps somehow? The other option I've used is Link2SD to a separate internal partition, but that doesn't work well with Whatsapp either.
Do I need to make sure I'm mapping the actual path to the internal/external storages and not symlinks/aliases, and in that case what would the actual path be?
Android/data/<package_name>) on the SD card, which led to a kind of sh,tstorm. So with LP, they opened that a bit more again – but now the app must use a specific API to access the card. Not being aware it accesses the card, an app is unlikely to use that API. So you could try linking to the one I've mentioned for KK in the hope it's still available to all apps.