Well that constraint goes without saying, right? My point was about the pre-conditions for the pandemic, not the knowledge available to a government responding to the pandemic. If a fraction of the 2017 manifesto had been implemented this would presumably have had implications for pre-pandemic social conditions (viz in relation to health and social care) regardless of the extent to which you think those policies were desirable in their own terms. So the interesting question, as far as I can see, is what those implications would have been. Imagining a substitution at the point of impact (so to speak) isn't a particularly interesting exercise. In fact I suspect they probably would have done worse in some respects if leadership was substituted at the final stage given the hysterical reaction that even a PM widely regarded within the establishment as a 'libertarian' (ffs) has provoked through unavoidable public health measures.
I dont imagine health and social care would have been in sufficiently good shape after that amount of time to make an immense difference, almost regardless of how good the policies were on paper.
I consider it incredibly unlikely that they would have invested in mass testing system infrastructure in the years running up to this pandemic, for example. I'm not convinced getting better PPE supplies in warehouses would have been the priority either, unless they took special interest in previous pandemic exercises when they took office. Nor do I imagine a revolution would have happened in the care sector. Nor would NHS capacity, especially hospital capacity, have been changed radically in a few short years, even with investment it takes time to make a difference where its needed most - staff.
What would have been possible is that these larger, medium-long term agendas would have been a much better fit for some parts of the pandemic short-long terms response. Emphasis on spending a greater proportion of GDP on health can fit well with the effects of a virus like this. Financial support for people to self-isolate would also fit well, etc etc.
Its a big stretch to think of these things because Labour didnt win. If they had actually managed to get in on an impressive reform ticket then they would have had momentum and presumably aspects of the establishment would have begrudgingly climbed on board to at least the extent of accepting that there was popular support for such a programme, that there was some legitimacy to at least some of the agenda. And then yes, when the pandemic came along some things could have been tied in quite neatly, we could have some cohesive policies with a long shelf life and somewhat consistent rhetoric. Your point included mention of how the media would have reacted, and to guess at that I would need to have seen the media in an era where Corbyn was actually capable of winning and did so, and then we'd have needed to see to what extent they were actually prepared to stick to the manifesto.
Why stop there if we are going to investigate hypotheticals though? We could go back further and get rid of the weaknesses that were made worse by the austerity years. Or go back further still and prevent decades of declining hospital bed etc resources. I'm not sure how interesting an exercise that really is either, because without the pandemic ever happeneing we could have reached much the same conclusions about the merits of having those policies all along instead of the shit we got for decades.