So I will respond to some of this:

‘Life started instantaneously after the Earth became suitable’. You may be disagreeing with my phrasing rather than the book's explanation here. I think Hoyle’s fundamental point was that if abiogenesis is such a difficult and unlikely event, why would it have happened ‘as soon as’ a suitable environment existed? (And if this is the case wouldn’t it happen all over the universe anyway? This one is my point, not Hoyle’s).

A while back some experiments were done showing that precursor biomolecules were able to be spontaneously generated from an environment similar to one found in the early earth. So there is some president that spontaneous generation of the early building blocks of life are possible. These molecules do seem to want to spontaneously assemble. However the conditions in the early earth were not conducive to the formation of the more complex molecules (IE long RNA chains... etc.) but that doesn't mean that these sorts of molecules weren't self assembling and falling apart over and over again until one day when the environment became more suitable for their stability. At this point these sort of moleucles would have seemed to appear almost overnight, all at once. Conditions occurred that provided stability to processes that were already occurring but not able to fully form to complete and stable states. This plausibly justifies why life quickly happened as soon as a suitable environment existed.

the clincher for panspermia is always going to be the confirmed discovery of biological or genetic material either in a meteorite or on an extraterrestrial body

I don't think that confirms much of anything. It seems to be fairly obvious that the molecules responsible for life here on earth are energetically favorable to form. It wouldn't surprise me if these sorts of molecules were able to form in other places in space as well. I am not saying I don't think panspermia happened, I am just saying I don't know that the observation you are looking for would conclusively prove it.

I wrote a short post about the "RNA World Hypothesis." About how life could have evolved out of early simple organisms that were actually just catalytic RNA molecules. This is another fun theory about how life may have begun, and their is some precident that an RNA world may not be entirely a throwaway concept (I mean consider that the complex responsible for protein synthesis, the Ribosome, is largely a catalytic RNA complex, with a bunch of structural proteins).

At the end of the day it is going to be interesting to see as more evidence comes in regarding the early stages of life on Earth. Perhaps we really were seeded by advanced alien life, or perhaps we were seeded by random chance, or perhaps life on earth is the first life in the universe, or perhaps there is life all over the place and it is independently existing due to the properties of the elements that result in the biomolecules that create life. I do not know the answer to any of this, however it is fun to think about.