bitcoin will survive any literary any price even at 0 because of the way it is designed.
you can search more about difficulty to learn more about it but basically the profitability of mining and how much it costs depends on difficulty and difficulty itself depends on how many miners or more accurately how much hashing power is mining bitcoin. and that changes with price, it is not a fixed amount. for example if price falls to a point where it becomes unprofitable to mine bitcoin and miners were put under pressure then some miners will stop mining and difficulty will decrease making it profitable again.
miners don't all have the same cost. some have cheaper electricity, cheaper labor,... are in cooler environment so need less cooling so less cost,... some have reached their ROI already and some have started recently,... so you see not all are going to suffer the same from same size of drop. which is why only some leave and difficulty changes a little at a time.
so far hashrate is rising despite the fall which means miners are making a lot of profit that makes them increase their hashing power instead.
Yes - but some parts I think no. Bitcoin wouldn't necessarily survive an extended period at zero because there will always be some costs associated with mining (having a cost to mining is part of the point).
Also the market demands that over a long enough time span we'll keep moving to an overall market breakeven point. This means, on a market level, the COST to produce/mine one unit of a cryptocurrency will be equal to (or at best perhaps slightly less) than the PROFIT generated by that one unit of cryptocurrency.
What differs is the cost at an individual mining operation level. This means that miners that can reduce their costs the most are the ones that will stay the most profitable and expand their operations. So the competition gets more fierce, difficulty goes up, profit margins get squeezed, smaller miners drop out and mining becomes more centralised. This is why large bitcoin mining operations are the most profitable, and everyone else is stuck using nicehash to mine some brand new shtcoin to swap for bitcoin
But anyway, on your point about if Bitcoin was valued at 0 (for an extended period of time), well, then there wouldn't be much of an incentive to mine it and no one can reduce their variable or fixed costs to below 0.