If so, then there is no need to have a third booster shot.
Why do you think this? Consider chicken pox/shingles. Clearly, infection with chicken pox once leads to immunity lasting for many many decades. However, that immunity can wane, allowing shingles to develop. Immunity is not an all or nothing status.
Also, consider which specific variant is used ...
Correct, in almost all cases, a fever should be left to run its course as it is a function of the bodies immune response.
Taking paracetamol to lower your temperature by a few degrees has a suppressive effect on the immune system response and therefore, the illness can be prolonged.
In rare cases does a fever reach such a high level that can cause permanent ...