If in an area of silence, can a warpriest still cast spells by using fervor, or would they need to use silent spell metamagic feat? What about using their blessing abilities?
As mentioned already, fervor only removes somatic components, not verbal... as for blessings, that depends on your chosen blessings... most of them only have a somatic component (touching an creature or object), if any, to their activation, however there are a few that do explicitly require speech and a couple more that your DM may rule requires speech. But for most blessings you should be fine just read how the specific blessing works and you should be able to make a good judgment call from there.
For example:
Nobility, the first level ability granted by this blessing requires that you speak a few words for it’s activation. The 10th level ability on the other hand stated that you inspire tour allies to follow your lead, by strict RAW it is unclear if this requires a verbal action or not, but it is not unreasonable for a DM to declare it a verbal action.