@saraclara I've heard elsewhere it can be confusing to kids to see parents swapping pleasantries at handover one minute but knowing they avoid each other at other times. Not saying you should be rude to each other, however the ideal would be to avoid any interaction at all. That way the kids don't feel they have to choose between one parent or the other when they're together, however briefly.
OP: I used to be in the same situation - I think my ex thought if she could just be pleasant and nice somehow I'd just forget her lies and gaslighting and "get over it". In truth, some actual contrition on her part might actually lead me in that direction. But, as is, the best she can offer is "I'm sorry you feel hurt by what I did".
It might get easier as your DS gets older: my DSes are now 5 and 8 and tend to run ahead, so I took to parking a distance along the pavement and letting them run to the front door themselves. Got in the car, made sure the door opened when they rang the bell and then left. When my ex does her drop off, I leave the front door ajar five minutes before hand. That means DSes run straight in the house. To be fair my ex at least has the sense to a) pick up on my behaviour when I drop off, to know I might not appreciate her saying "Hi" and b) if not, since the boys are greeted by my in the hall before my ex can make it to the front door herself, I can get away with vague nod and wave before shutting it.
I guess some might thing that sounds cold but a) it isn't half as chilly of what she did to me and b) it, quite frankly, simplifies things greatly. Everybody knows where they stand. The boys know Mum has her life and Dad has his. There's no pretence we're still a "family". And, curiously enough, its made me feel more accepting about our lot now. I finally feel free of the deceit - because in some ways, all that "hey, despite throwing you under a bus, we're still a family, right?" behaviour is still a lie.