Haven't played in a million years but can someone remind if Rowan HAS to fuck Heleyna or whatever her name is for Alexia to get onto the Andras route? Because it seems that without doing that her favorability with Rowan never dips low enough for all the Andras events play.
The Andras NTR path is triggered by meeting the following criteria: A low relationship between Alexia and Rowan, Alexia having at least medium corruption, Andras having more influence on Alexia than Jezera, and not already being on Jezera's NTR path. If you don't meet
all those criteria then you wont start the Andras NTR path.
The first NTR path scene is the one when Rowan asks Alexia to deliver some orders to the orcs and you have to take certain actions to go down the NTR path: when the orc insults Alexia have her insult the orc back, after the orc almost rapes her have her turn to Andras for help instead of Rowan, then have her agree to meet privately with Andras afterwards.
Once you have done those three things you are on Andras' NTR path. Warning, once you are on his NTR path you will have limited choices when dealing with Andras' NTR events going forward.
Note that there are a good number of Alexia x Andras scenes that are not part of the NTR path, and are just regular cheating, but they still are useful for increasing Andras' influence and increasing Alexia's corruption in order to trigger the NTR path.
As for lowering their relationship, in addition to sleeping with Helenya (which depending on how you talk to Alexia about it may be a major or minor drop):
-You can generally have Rowan be dismissive or or jerk towards her (there will eventually be plenty of opportunities).
-You can also get a big drop when Helanya comes to Rowan for help escaping, if you refuse to help her escape Alexia will help her instead, and if you turn Alexia in it will be a huge hit.
-If you are sleeping with your elf secretary, eventually Alexia will confront you about it, and you can get a decent drop if the encounter goes poorly (or a threesome if it goes well).
-If Alexia is working as a maid there is a fairly common repeating event where the other maids will be gossiping about Rowan supposedly sleeping with a femle castle NPC (these rumors will exist regardless of whether or not you down anything), and if you have Alexia let the rumors get to her the relationship will take a small hit each time, and it is a slow but reliable method especially if you don't want Rowan to be a complete bastard to his wife.
-There is a similar event when Alexia is working at the tavern, if Rowan has had at least 2 gay sexual encounters, there will be a repeating event in which Alexia questions his sexuality that gives the relationship a small drop