Was this the first year in Oscar history that Best Actress came after Best Actor?
No...this happened in 2012 (84th) and 2015 (87th Oscars) as well. There may be others..
Is there a known plan for how to order actor vs. actress awards in the future (such as alternating the order every year)?
There does not seem to be a fixed schedule of alternation.but the final four are usually
- Best Director
- Best Actor / Actress (as stated no set order)
- Best Actress / Actor (as stated no set order)
- Best picture
Do you have a source explaining how/why this decision was made?
No, and there doesn't seem to be anything on the Academy site as to the order rules but you must recall that the Oscar ceremony is a production and so the announcemen order is a part of that.
I would suggest that the order of the awards (which is announced before each show) is a part of that production so it seems likely that the order of the Best Actor/Actress is such that it will have the most production value.