Adjectives can inflect when modifying nouns, but it changes the meaning of the sentence. おいしかった食べ物 sounds like the food was delicious, perhaps until you ate it (at which point it was no longer delicious). I think the present tense just sounds more natural for your intended meaning of “I ate delicious food.” Note we don’t need extra past tense for “delicious” in this English equivalent either. We can say in English, “I ate food that was delicious” but that’s a matter of agreement (the adjective “agrees” with the tense of the subject) whereas in Japanese I don’t believe that is a thing. At least not in this case.
を is used to mark direct objects, whether they are people or things. You can say 人を殴る “hit a person” for example. The reason you use に for this type of sentence is because it’s acting in a similar fashion to the word “with” in English, when we say “I met with my father.” We can say “I met my father” in English, but in Japanese with 会う it expects you to mark the person you’re meeting with with に.
Edit: also what @alo said about 会う being intransitive is correct. That’s why it’s ungrammatical to use the direct object を marker with it. (This is getting into more advanced stuff, but sometimes を is used for other purposes besides marking the direct object, in which case it may be possible that it is used in a sentence with an intransitive verb, but again that is more advanced and not really relevant here. Just felt I should mention for completeness.)