Well, Olivares was doing the same thing on Aragon, which triggered the temporary secession of Catalonia and its temporary union with France, and also a small rebellion in Aragon.
A less harsh (i'd prefer more realistic) Olivares would largely be more in tune with the policies of Lerma... but he despised Lerma and blamed him for the decline of the Spanish power.
If we want to keep Spain united, we need to find another way for Olivares to channel his reformer spirit. Or give him a counter-balance that could moderate his policies.
Checking the wiki page on him, i see a couple of potential PoDs: on one side, his uncle de Zúñiga as foreign advisor was important on his decision to restart the war against the Dutch. This was probably an unnecesary war that deprived of resources for the wars later on.
Also in 1628 the treasure fleet was captured (by the Dutch) though an insolvency crisis, which caused him to press Portugal for higher taxes, which in turn provoked the first attempts of rebellion.
OK HUGE amount of time between the Portuguese succession war and the War of Spanish succession. With such a POD, the Spanish Habsburgs might not go extinct, so no succession war.
By the time Spain stops trying to retake Portugal, Carlos II is already king (even if underage). Even by the time Spain stops having real options to get it back, it was a sure deal that he was going to be king. And as long as Carlos II will be king, there will be a successional crisis: what could possibly prevent the rising star of France to project its influence over Spain, specially a Spain ruled by a weak of character and sickly king? With a PoD near Carlos II crowning, i think a Succession War is very likely.
However, in my opinion, the Iberian Union was pretty doomed from nearly the start. Under the Union, Portugal lost large amounts of Colonial possessions, went to war with England, its traditional ally, and entered into a useless war with the Netherlands. So unless Spain can make the Union profitable for Portugal, sooner or later the Portuguese are gonna go, Hey whats the point of having a foreign King if we keep losing our sh*t? Plus, it was a union at the edge of a sword, not the best beginning for something like this.
Portugal was not in a very different position than Aragon was, regarding to Castille. And while there was rebellions in Aragon, also supported by the foreign powers, they ultimately failed, so, there's no reason a priori to think that the Portuguese rebellion was bound to succeed.
Also, the union of Castille and Portugal was hardly "at the edge of a sword". Most of the Porguese nobility had supported Phillip II's claim.