Lou was 100% chosen for ryan. Like you said they had far more chemistry than lou and oliver ever had, which is why I could never warm to their on screen relationship. There was zero chemistry, where was the warm, the heart eyes, the little touches. I know lou wasn't on screen much but still, it was like they were complete strangers
There are shows that even with a very limited screen time, can make the audience fond of a relationship.
Ashley and Bradford in The Rookie comes to mind.
And on that subject Lucy and Chris, Jackson and that actor we barely saw but they were fun together.
Casey and that cute reporter from Chicago Fire, Britney and Severide were short but bizzarely cute.
I can give a lot of examples, Eli the Station 19 bi character was chasing Andy and the fun of it made me root for them but eventually he slept with Travis, and I loved them too.
There is even a funny bit in the end of S6 where Eli dating Travis compliments Andy of how good she looks at the Gala and she quips with I know.
There are so many ways to make a relationship work even for small stretches of time, but there should be a proper chemistry test, which I would bet money was done between Ryan and Lou, but wasn't made with Oliver and Lou (kind of like the Natalia actress failed the chemistry test with Oliver and they still brought her on) and it shows, the actors were not invested and the writing was horrible.
No director even bothered to shout at them to fake it lol
Dean Kane and Terry Hatcher hated each other but Lois and Clark was a success for many years, because they knew how to fake it well.
There was none of this in 911 and it was to the detriment of this arc because if could have been a blast.