Contrary to common belief, the words "they" and "their" have been used as indeterminate singular pronouns for centuries in English. If you believe in prescriptivism in language then the use of "they" as a singular pronoun is perfectly correct.

However, in the above sentence "her" would have been appropriate. :)

I don't think this usage sounds awkward. It would be used when someone was in the middle of a project (for example) and wanted to give an update on a portion of it being completed.

"The group has 12 tasks to complete by Friday. As of Wednesday, we had completed three tasks. We will need to get four tasks done per day to finish the project."

"Someone else's" is correct if you are referring to an item that belongs to someone else. E.g., "This is someone else's blog entry."

The correct plural for "passer-by" is "passers-by". This is because the word "by" is being used as an adjective modifying the noun "passer". The noun is the word that is being pluralized.

In most cases in English the adjective goes before the noun, but in compound nouns such as "passer-by", "attorney general", and "mother-in-law", the adjective is used to modify the noun, so it is not pluralized. I've found it helps to mentally treat the adjective as a separate element from the noun, or place it in the usual position for an adjective, when trying to determine the plural. Thus we have:

One passer-by (one by-passer, one passer going by)
Two passers-by

One attorney general (one general attorney)
Two attorneys general

One mother-in-law (one person who is by law my mother)
Two mothers-in-law

I don't believe "quality control" can be used as a verb. A hyphen would be used to turn a noun phrase into an adjectival phrase. For instance you could say, "We performed quality control on these widgets. They are quality-controlled widgets."

I believe the statement you quoted above should say, "As long as we perform quality control on questions, we should not have to perform it on comments."