Q12 (b): I have entered all information on my application. However, the "Tax Year" boxes remain blank and aren't populated. Is this a bug? What am I supposed to do?
You should fill the other parts of the form (especially the part that talks about your eligibility period and only then will the 'tax year' rows will be populated.
Q14: I have two passports (one valid, on expired) from the same country. Neither has a "Place of Issue". Instead, they list an issuing authority (i.e. municipality where I resided, and Ministry of Foreign Affairs for my current passport). What should I enter for this field?
What do you see on your immigration document (COPR)? Enter the same thing. AFAIK, it would be okay to enter the issuing authority but check your COPR just to be safe.
Q14: For my previous passport, is the Expiry Date the date when I obtained my new passport (and they "expired" my old passport). Or is it the original Expiry Date when the passport was issued (and listed on the biographical page)?
Enter the dates exactly as shown on the passport (not the date your passport was 'cancelled'). You can add a Letter of Explanation and explain the scenario if you see the need.