You won't get any arguments from me. As far as I'm concerned SN 54.9 definitely rules out full omniscience.
My possibly simplistic take is that the Buddha 'knew everything' by knowing sunnata, the inherent emptiness of all things. He knew everything conditioned as sunna, anatta, anicca etc. and he knew the unconditioned as nibbana. What else is there to know?
"Do not force others, including children, by any means whatsoever, to adopt your views, whether by authority, threat, money, propaganda, or even education." - Ven. Thich Nhat Hanh
"The uprooting of identity is seen by the noble ones as pleasurable; but this contradicts what the whole world sees." (Snp 3.12)
"To argue with a person who has renounced the use of reason is like administering medicine to the dead" - Thomas Paine