I think read-only is what most businesses would want, that seems to be how most BI works.

Other challenges:

1. Building uniform data connectors suck. GLHF.

2. Long queries: can you return a promise, then callback to an Apps Script endpoint when the data is ready?

3. Row Limits: I don't see aggregation in your screenshots, that would certainly help. Spreadsheet users know about row limts, just give them good UX and control. Sheets gets sluggish pretty quickly on large data sets, so you may want to have a lower default limit so the UX stays snappy and a warning for large queries.