Is my pan too hot for brown sauce / gravy?
The solution is very simple.  


Fry steak 
Saute onions 
Remove from stove, wait a minute or two 
Melt butter, add flour 
Season with heat insensitive stuff (e.g. salt)
Add stock 
Return to stove 
Let simmer for a couple of minutes 
Season with heat sensitive stuff (e.g. fresh tarragon) 


Assuming that you are frying your steak below the carb charring temp of ~190°C, the minutes spent away from the heat and the heat spent to warm the butter and the flour will change the mixture's temperature to something much more maneageble, below the smoking point of butter (roughly 150°C). It will still stay above 100°C, but the steam building won't be too terrible. It is normal to prepare roux at above 100°C. 

As for the damage, I don't think that we know it. I asked about it once, but no answer was complete enough to accept. See How does thermal shock affect pans made of different materials? for details. Still, I don't use coated pans (PTFE or this "diamond" stuff) for deglazing. The 400° proof ceramic coated ones could be safer, but I still find steel better for this application.