First of all, welcome RRR

I'll try asking your questions properly.
1. Window_Message y is decided here: class Window_Message, line 170 on:
CODE
def reset_window
if $game_temp.in_battle
self.y = 16
else
case $game_system.message_position
when 0
self.y = 16
when 1
self.y = 160
when 2
self.y = 304
end
end
if $game_system.message_frame == 0
self.opacity = 255
else
self.opacity = 0
end
self.back_opacity = 160
end
0 is referred to the top window position,
1 is referred to the middle window position,
2 is referred to the bottom window position.
Modify the y values here to move your window where you wish to;
2. Sure, you can resize the message window here: Window_Message, line 12:
CODE
super(80, 304, 480, 160)
Just modify that 160 with the number you like

Remember that 32 pixels are always lost because of padding.
Anyway, remember that this solution will crop away the exceeding lines of text.
To use all the lines and then subdivide the text into two windows, you'd need a more refined script.
3. Gallery items belong to the user who opened it. To use them in commercial games, you have to ask the submitter via PM. Different people could have different reactions about it.
I hope this could help,
Jens