I wonder what made your mechanic think that throttle body can be cleaned by high octane. What you did on the motorway probably cleared some carbon deposits and freed up the exhaust.
If reservoir bottle was at same level, then you're right about the faulty cap(bad relief valve). The bubbles seen may also be related to radiator cap as it was not making pressure and the water/coolant started boiling. Ideally the water level should rise slightly then back off once the car cools
Which coolant was used?
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