a) Yes. I have 10 or so certificates (dev & dist) on my computer for various clients. I keep them in separate keychains for peace of mind. When it comes to time distribute your in-house binary, you archive in Xcode then hit the Distribute… button in the Xcode organizer, choosing the correct Enterprise profile.
b) This is no longer necessary as the Distribute… step mentioned above embeds the profile in the app. Things are much easier than they used to be.
NB: I avoid wildcard provisioning profiles as they can cause heartache, even in simpler situations than yours (e.g. if Xcode chooses a wildcard Ad Hoc profile during Archive, then your entitlements may be wrong once you Distribute), so for this reason I recommend you always use explicit profiles.