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Your interpretation of the GA4 metrics seems mostly correct, although it is always important to consider their context in actual usage. The metrics you found later (campaignName, source, medium, manualAdContent, manualTerm) are dimension items usually associated with a specific event (like page_view or ad_click), whereas session prefixed dimensions (sessionCampaignName, sessionSource, sessionMedium, etc.) are associated with user-level or session-level data, rather than event-level data.
Coming to the Multi-Channel Funnels Reporting API, unfortunately, GA4 does not support Multi-Channel Funnel reports in the same way as Universal Analytics. Instead of using the Multi-Channel Funnels Reporting API from Universal Analytics, GA4 proposes to use the new ‘Conversion Paths’ report, accessible in GA4 interface, although it doesn’t have an API as of now. However, while this may not exactly have the flexibility and sophistication of the Multi-Channel Funnel API, it does provide a way to explore user conversion paths similar to the Multi-Channel Funnel Reports.
