Majorca training camp/bike hire

Is it, like, uphill pretty much all the way to the Coll dels Reis?

yes

I'm kind of worried nothing is going to be open (snack bar/restaurants, etc) when I'm attempting it, so am a mite concerned about hydration

unsure if anything will be open right at the top of sa colobra at that time of year, but there is a petrol station by lluc you will cycle past. its a repsol iirc so will definitely be open