Lavoisier’s errors you mentioned were not entirely unfounded. His contention that what made an acid was universally oxygen. At the same time, English scientists asserted that acids universally contained hydrogen. They were correct.
In practice, Lavoisier’s was not an unreasonable assumption, as the great majority of common acids at the time contained oxygen, and because it is such a noteworthy and reactive and reductive ion. Only with the later discovery of hydrochloric acid did it become plainly clear that oxygen was not the essential characteristic of acids.
His other incorrect assumption, that heat was a substance, was likely due to an entirely new state of matter unknown at the time: plasma.
Plasma can most definitely act like a substance, which would lend itself to the impression that heat, not the invisible plasma, was exerting pressure. This would appear to be the case in Lavoisier’s time. Proof that heat was not a substance would be very difficult.
What? This makes no sense. He was quite familiar with the common understanding of heat, and of course expert in its laboratory applications. He gives careful explanations of the common experiences of heat in terms of the passage of caloric between bodies.
I don't know why you would want to attribute the idea of caloric to some experience or anticipation of a plasma state. The closest thing in our modern understanding to caloric ( i.e. the concept of heat as a fluid substance ) is enthalpy, the thermodynamic state function with symbol "H", which we always jokingly said stood for "Heat".
The point is that "caloric" was in our terms a misconception of which, for very understandable reasons, Lavoisier was never disabused, in spite of his belief that he was dealing in Facts of Nature.