1) The changest will stay in effect as long as the instance is UP, IF you use scpoe=memory when changing the parameter. The default is BOTH what means it changes the SPFILE as well. I can hardly imagine that you can avoid using SPFILE if you are not doing that by purpose. The default database even creates SPFILE
2) Mounting the database have nothing to deal with the alter system command and the parameter changing. The changes are in effect until you do not restart the instance if they are done ONLY in memory. If I do
the changes will be in effect although the database IS NOT MOUNTED
3) There is nothing wrong to say "I am wrong, sorry, I just retyped the documentation but looked at an old one"