I think situation 2 is not that bleak. We can find a lot of very good young talent to replace players. They won't necessarily be top 4 contenders next year in that case, but will get you out of your seat. Most clubs in Europe are facing very bad financial situations and this summer will have a lot of forced sales to balance the books (especially in France).
Given that we need to overhaul the squad anyway, if we can take advantage of the situation to stock up on good players while focusing more on getting rid of bad players than getting good prices for them, we could give the new "project" manager a much more positive situation even if Kane or Son were to leave. The other plus side is if you buy kids who didn't go to the Euros, but instead had a proper summer break, you could have a fitness advantage next year over teams whose players did play in the Euros. Especially given the way the schedule gets ruined by the World Cup next year.
I don't know that we're smart enough to do this, but the potential is there to do it, and it would not be a disaster.