Depends, I don't really think that giving Saber back Avalon would, in most scenarios, be of much value as she already has pretty high regeneration capabilities, while Shirou has none, thus it would be of a strategic advantage to have one person with really high durability and resistance and the other person pretty much immortal than to have one person pretty much immortal and the other person close to defenceless, especially when their plan is to do a double-confrontation, very likely exposing both to rather high degrees of danger. The route options obviously did not favour Shirou keeping the Avalon, however, I would say that realistically it would've been a preferable option, unless Shirou being capable of projecting Avalon on his own was the plan all along, in which case it would seem too risky of a plan to suceed.
At th end of the day, she had great intuition, it wasn't innocent kid's lack of understanding of things like love and marriage which makes some little girls say things like "I'll marry my daddy/brother". No, she noticed it was the beginning of something serious and unhealthy. She failed to stop it though, no matter what she did.