That's exactly it. He knew the true nature of the fruit. The WG wouldnt be putting on such an extreme guard for an ordinary or "weak" devil fruit like that.
No way he wasn't looking for Ace and to groom him for the Nika fruit to become Joyboy too. Shanks probably didn't want to eat it himself ofc or he felt Roger's son would be more worthy/fitting or a "better sacrifice" or something idk.
Maybe it was Roger's last request to him somehow?
There's still the matter of how Rouge kept Ace in her womb for what 2-2.5 years or w/e? She's a D and yet we still know NOTHING about her, she didnt even get panel time in the Oden flashback so surely Roger will get another flashback ofc and we will see him meeting Rouge, falling in love and why he has a sword named Ace and thus named his own son that way too? Right?