Is the hair that falls out as a result of catch-up-hair loss lost forever?

There's good chance you'll see an improvement and hopefully that improvement will come close to what things were like before. My experience is that hair regrowth can be quite good within three months but may not return to exactly the same as before stopping. 

Unfortunately each case is different. There is no way to know exactly how well it will grow back. I would recommend getting back on as soon as you can. Sooner you do this the better the chance of regrowth.

Yes, of you lose hair after stopping finasteride, the hair loss may very well be permanent unless you can restart the finasteride quickly