I did a side by side comparison between both the ova and episode 24. I repeated this 10-15 times to make sure. These are my findings:
During the first part until saber looks at her hand it is the same. After that it changes:
In the ova saber smiles and starts disappearing. She seems less keener to go though. Just as saber starts smiling Rin runs to save her. Rin shows a lot of determination to save her as well.
Episode 24: Rin completely forgets about saber. It is clear she's gonna disappear but she just mindlessly looks at the destroyed grail. Saber smiles and congratulates Rin, but Rin keeps staring at the destroyed grail. Saber is happier to go (her thoughts are "shrirou has you"). In the ova Rin starts running when saber smiles, but here she just forgets about saber.
Now that we have the facts clear let's answer your question:
- The archer question: It was already answered by someone in the other thread someone linked you in the comments. What was said is:
"Something I didn't mention before but is quite relevant is that Rin either subsconciously or conciously chooses Archer (Shirou) over saber. She asks archer to remain in this world and cries when he doesn't accept. This happens in both the good and true ending (right before the good ending would come archer's scene).Think that if archer had accepted Rin's proposal both the true and happy endings would be the same (Rin - Shirou - Shirou). I do believe in the good ending Rin runs to save saber because of how much Shirou cares for her (at least 4 points given to her). Even so by asking archer to stay afterwards saber would have left this world had archer accepted (there's not enough mana for both of them and Rin knows it. She also knows saber isn't selfish to stay that way, by asking archer she's in a way betraying saber, that's why she thinks "I know I shouldn't ask" (but she does ask...) and archer says "I do not know if you will keep your contract with saber, but I am nor qualified for that")"
- Why doesn't Rin save Saber in episode 24 of Unlimited Blade Works?
It would appear she forgets about her. If I had to guess I'd say that Shirou giving points to both girls plays a mayor role:
True ending: Having received all the points Rin forgets about saber for some time. Shirou can live happily without saber and Rin can do the same. You could say Rin's mind is telling her everything's ok.
(I'm assuming she never realises she's disappearing and that's why she's looking at where the noble phantasm was thrown. If she knows she's dying then she's a selfish BITCH).
Saber is also happier to go. She knows she doesn't play a big role in Shirou's life.
Good ending: Rin's mind works perfectly and she quickly makes a dash towards saber. Rin knows Shirou needs saber and she likes spending time with saber as well. Her mind and body work at its fullest to saver her.
Saber, knowing Shirou's love towards her, accepts. This is actually quite important. Both contitions are in my opinion relevant. If saber hadn't felt she's needed by Shirou she would have likely chosen death even if Rin had cried and begged her to stay (just like she did with archer)
My conclusion: In the end the one that plays the mayor role is Rin. I do believe she doesn't even remember saber cast her noble phantasm and is dying. You have to think that Rin can't just look at saber and say: hahahaha die you saber bitch!
Rin forgetting about saber is most likely the best option to show she cares a bit less about her but at the same time allowing her to keep her good nature (had she remembered she's dying just a few meters away she might have run to save her).
Then again it may also be that in her heart Rin accepted Shirou and her should be together just the 2 of them and purposely did it. We'll never know. I want to believe saber becomes less relevant and Rin just forgets about her, it's a much better interpretation.
Another valid theory is hesitation. Rin hesitates for a few seconds, just enough for it to be too late to save saber.
Just rest assured, she's not a cold-blooded murderer. If it were up to her I'm sure she'd rather everyone lives happily. The ova just shows that the good ending Tohsaka doesn't hesitate for even a second. The reason? It has to be that Shirou cares more about saber cause everything else is the same (Rin and saber have the exactly same relationship in both the good and true endings).