There is a lot here that you wrote and I do not presume to even try and answer you point by point. To start on a light note: “Why is there a shortage of loo paper?” “Because every time one person coughs, a hundred shit themselves”.
But let is analyse some of what I perceive to be known about SARS Cov2 and Covid 19.
SARS Cov2 is the recently described virus which has been associated with a set of clinical conditions from being completely asymptomatic, to having mild cold like or flu like symptoms, to being severely ill, to requiring ventilation support and at the extreme to dying of multiorgan failure.
Covid-19 on the other hand is the disease caused by the virus which constitutes only about 20% of all those who encounter the virus according to several studies. So not everyone who is infected with SARS Cov2 develops Covid 19, in fact 80% go around their business merrily and do not even notice. This is not unique to this virus because disease caused by many agent, whether microbiological, neoplastic, inflammatory or chemically induced, are manifested by the combination of the presence of the agent, the amount or dose of exposure, the timescale of the exposure, the state of the immune system at the time of exposure, genetic makeup of the individual, dietary factors and so on.
In the case of Covid 19 some observations so far have shown some of the factors associated with severe disease. There is a very strong correlation with age and certain comorbidities, including heart disease, diabetes, factors such as obesity, ethnic origin and sex. These all contribute to the variability of the disease in any one group of cases.
What has also been known from the outset is that the most extreme end of the spectrum, respiratory failure is associated with what is medically known as ARDS, whereby fluid accumulates in the lung and the patients literally drown in their own secretions. Ventilation in these cases has a limited effect because you can’t force oxygen through all the inflammatory fluid in the alveoli, but is used purely as a holding measure. It has been shown, in the case of Covid-19 and also other diseases associated with ARDS, as well as multiorgan failure, that this situation is caused by an overreaction of the immune system (known as a cytokine storm) to the virus that occurs in some cases. Why this is the case, is not known. One of the practical outcomes of this knowledge is that dexamethasone, an immunosuppressive agent, used in medicine to treat various conditions associated with dysregulated immunity, but also cancers of the immune system, has been shown to have some beneficial effects in patients at this severe end of disease increasing their chances of survival.
Also unlike some expectations, some patients with immunosuppression due to their cancers were found to be surprisingly less susceptible to severe disease.
To go back to some of your points and calculating the possibility of finding a positive or negative results. This is in the realms of experimental or theoretical mathematics and may be used for algorithms or other guidelines in public health planning. But in the glaring daylight of frontline medicine the positive PCR for SARS cov2 means with a 100% certainty that you have the virus in your secretions. The test has a very high degree of specificity because it is looking at unique sequences and amplifying them. False positive results arise nevertheless mainly because of the possibility of contamination because of the high sensitivity of the test and hence the 5% error rate. Nevertheless in a well controlled laboratory with rigorous procedures the a positive result should mean that the virus has been detected in that individual. Rigorous internal and external quality controls are used in order to validate and accredit laboratories.
So moving on to symptoms of covid-19. I think there is a subtlety that escapes the layman, say about certain symptoms. To take an example, loss of smell and taste. The mechanisms that operate in this symptom and in covid-19 are completely different. It has been added as a specific symptom, because it often predates any coryzal symptoms, such as sneezing or a bunged up nose, and even occurs in the absence of these. In the common cold, loss of smell and taste is relative, and is due to inflammatory secretions around the taste and smell organs, but in covid-19 it appears to be due to an effect on the nervous system that transmits the perceptions of taste and smell from these receptors to the brain. Also the cough in typical covid-19 is often dry and associated with breathlessness much before there are any other respiratory symptoms, unlike in flu for example, where the shortness of breath, due to secondary bacterial pneumonia happens later in the disease.
Just to take one or two other points you make: The care homes death from covid-19 was at the early stages markedly underreported and there was no testing of care homes or even of elderly patients in hospital with early discharges to care homes, which then led to this tragedy of care home deaths. To mix this up only gives leeway to Johnson and co who want to absolve themselves of any responsibility.
The other point about statistics and % positives I wish to make, which I am sure you know, is that the positive rate is a shifting one because of the variability of number and symptomatic groups of people tested, but also because of the shifting infection rates. This moving target makes it difficult to make too many assumptions at present.