If you get your visa renewed at migraciones in country and then leave & return before that date expires - the officials at the border CAN NOT override that initial date. It has always been the case (or at least for the past few years). It is nothing new. So if you go to immigration today, get a 90 day renewal and then leave after 45 days, you will not get another 90 days when you return. Your original date will remain unchanged and you will have to leave at the 90 day mark again.
Again - for all the doomsday scenarios, I know multiple people who have continued to go about their business of being a perma-tourist with no problems.
There is no guarantees, the individual agents have the right to guess & ask questions but at least from the people I know, they have had no problems.