3. the immunity gained through an infection holds for a long enough time (how long would need to be checked with the infection rate), which we do not know yet

Additionally, we know nothing of the long term effects even for the "young and healthy". Mortality rates for people 10-39 is 0.2%, 40-49 it's at 0.4%, 50-59 1.3%, 60-69 at 3.6%. Serious conditions develop in a bigger proportion of cases, so you are looking at 4-10% of under 60 population in ICU with unknown long term effects.

Translation: good luck!