If 90% of the computational power was stopped being offered to the Bitcoin network, then it depends on the block that it'll happen. It'll surely slow down the process independently of the block, but if it happens 5-10 blocks before the next adjustment, the damage won't be that big. Each of these blocks timestamp would differ around 200 minutes. But if there are thousands of blocks until the next adjustment, there's trouble for sure.
Yes, it will since that's not how the difficulty adjustment works.
If you experience a drop of 90% in hashrate but only on the last 5-10 blocks the difficulty for the next week will still be nearly on the same value as its average of the last 2016 blocks, so you will still have the next two weeks period to deal with it till it adjusts 90% down. Same if the difficulty will drop in the middle of the period, you will only have 7 days of blocks being 10 times slower but also another two weeks of them being 5 times slower since the next period difficulty will be half of the original but still 5 times higher than how it should have been considering the last 7 days.