If you only have one very high level Pokemon(starter or other) that's ok until lance. i recommend leveling up ur weaker Pokemon if u get stuck on lance, or have lots of revives for ur starter.
source: experience( my starter is still my only high level Pokemon)
no beat the elite four first
You never go to Hoen.
Yes you do
Yes
Yes they do I can tell you from experience.
nothing
go to the kanto region
They are still in the Pokemon league
yes you can but you have to beat the elite 4 first
When you beat the elite four.
elite 4 or mt silver
Yes. To unlock it, you must first beat the Elite Four.