In each case it steadily rose after case growth slowed and lagged deaths continued, as expected.Where do you see that? The data I see says the fatality rate of all forms has been dropping over time, as expected from prior epidemics where this also occurred. It's the Imperial papers that have been discredited by now, as their projections were way off in all respects - if they were right Sweden would be a bloodbath at the moment.
I agree the serology survey was not a perfect study - more for statistical/confidence interval reasons than the researcher's wife. But it may not matter. The general conclusions of the serology survey are matching many other pieces of data and evidence from around the world. For example:
"The latest figures from Italy show (pp. 12/13) that 60 of almost 17,000 doctors and nurses who tested positive died. This results in a Covid19 lethality rate of less than 0.1% for those under 50, 0.27% for those aged 50 to 60, 1.4% for those aged 60 to 70, and 12.6% for those aged 70 to 80. Even these figures are likely too high, as these are deaths with and not necessarily from corona viruses, and as up to 80% of people remain asymptomatic and some may not have been tested. Overall, however, the values are in line with those from e.g. South Korea and give a lethality rate for the general population in the range of influenza."
From https://swprs.org/a-swiss-doctor-on-covid-19/ - the source document is in Italian but the data table is quite clear even to someone who doesn't speak it. In other words for Imperial's paper to have been reasonable they'd have to assume the entire population had the mortality profile of 60+ year olds, which is just wrong.