1)
Both can be possible as I wrote "
(This can be invitation "Let's go shopping this afternoon", as you would know.)" in my previous post.
Japanese verb conjugation - Wikipedia, the free encyclopedia
2)
Yes, that's right. It's presumptive. (早い usually doesn't mean "fast", by the way. "Fast" is mostly 速い.) FYI, ない is also an i-adjective.
~かろう can't be volitional since adjectives express a state, i.e., they are always non-volitional.
from テルーの唄 in a Studio Ghibli film
Tales from Earthsea
悲しかろう = 悲しいだろう
切なかろう = 切ないだろう
寂しかろう = 寂しいだろう
手嶌葵/歌詞:テルーの唄/うたまっぷ歌詞無料検索