If I happen to miss a day of voting, so that I have two categories to vote on the following day, can I vote for one of those categories and then come back later in the day and vote for the other?

Yes, you can come back at any time during that day (NST) and vote on the remaining category (or, for those who miss two days, categories). If you miss three days, however, you'll only be able to vote for that day's category and those from the previous two days, since the three-day window for the category from three days ago will have expired.