Well a) is provided by Mongo and is literally the reason why Meteor can do exactly what you described: multi-user webapps which update in real-time.
Leaving aside reliability and ease of use, let's focus on correctness. RethinkDB lets me query a database, get initial data, and then get all the subsequent changes to that data. Notice there is no race condition there.
You can use this to implement systems where when a user logs in, gets the initial data loaded, and then subsequent changes are sent as they happen. Even if the same data is modified by someone else during this time (e.g. during the initial load), things will be processed correctly.
Comparing this to attaching a processor to a feed of all operations in the database doesn't make a lot of sense, because the oplog doesn't provide the same functionality.
Could you post a link please?
So I guess Mongo+Meteor match up with RethinkDB... sort of.
Obviously the performance characteristics will be impacted by the volume of changes that arrive to the database, but the architecture to support this is highly parallelized (all the way down to cores on the CPU.)