I suppose rather than Geralt actually witnessing the heated argument but seeing the fallout from the exchange & get second hand account of it from the LI.
I think the later acceptance scene would make much more sense to have all three present.
I think Geralt simply needs to tell them his choice with the one he choose present.
I think Triss knows that when he said he love her at the docks and begged her to stay with him his feeling were sincere but, she still has doubt because Yen is alway close by.
If Geralt talk to both of them together and tells Yen that Triss is the one he wants to be with and hes not willing to lose her with Triss present to see and hear him telling her that. All doubt will be eliminated for good.
Then they can finally move on to hashing out the detail about their life and future plans in Kovir.
I can understand other people thinking about Yen but even in the books although Yen and Geralt were couple I felt he had more chemistry with Triss for some reason. In all 3 games I felt the same thing too. Every conversation with Triss was lovely with Yen it felt like two people forcing themselves to have a decent conversation. Just my feelings tho.