Is it okay to ask to meet my potential engineering partner or future team before accepting an offer?

Yes, but route it through the recruiter. Ask specifically to meet the engineering lead you'd be partnering with — that request is usually honored. Asking to meet the broader team is more situational; some companies won't do it because the team may not know hiring is happening, or internal promotion decisions are still pending. Let the recruiter guide what's appropriate based on their company's dynamics.