My understanding is that the 1/3 rule is based on the last date you stayed at a Mayan property, not calendar year. You can have consecutive and multiple weeks if they are all part of the same trip and so that should not be an issue.
Not sure if it is "correct" but I have used my RCI points and weeks accounts as you are doing (as separate accounts) to stay more than once in three years. I assumed that if RCI would allow it that there was no problem, since the 1/3 business is only with RCI. My weeks and points accounts were not linked so not sure if the result would be different if they were linked.
I also recently booked into the Grand Mayan Riviera Maya (Jan 2015) using Interval, so it seems that there are occasionally weeks available for trade on II (no 1/3 rule if you use another exchange platform, it's only RCI). I have also seen them on other exchanges so you might also look outside of RCI if you have the ability.
Final point, even when the platform would not permit the booking I have successfully called RCI and gotten a waiver - this works best if it's not peak season, but I think you may be in the shoulder and so give it a try - I've been successful two times (about 50/50) so far in getting a waiver from RCI.
Lars