1. I would drive home and first give the old lady next door enough food to last the whole year. 2. I've already had a trial ride in that model you're looking at. 3. She missed the school she'd stayed away from for a long time.
So, could you tell me whether "I gave the old lady who lives next door enough food to last the whole year. " is fine?-- Yes, it is fine.
Also, if there is "which" between "school" and "she", like "She missed the school which she'd stayed away from for a long time.", is it okay?-- Yes, or 'that'.