Any question answered...

5mm shorter cranks won't hurt you, even if you notice them (which most people don't, in blind tests)

I have two of them on the Tester Tandem. They are worth the money, if that's your budget, then that's my recommendation.

Alfine is nothing like 105, that's why it's half the price. It's more like FC-4600 Tiagra, i.e. not hollow.

Thanks for the clarification! repped!