Howdy,
To the best of my knowledge, it depends :-)
If the port is configured as 4 virtual 10Gb interfaces losing "pair 1" will not impact pairs 2-4.
If the port is configured as one 40GbE interface utilising 8 cores of the MPO you need all 8 working as you'd expect.
MPO cables come in 2 types - straight-through (for structured) and cross-over for direct device to device connectivity.
BTW - have you seen the BiDi options and the AOC cables that are coming through? Lots of bandwidth for less bucks, need to check the cable lengths and see if they fit your use case. QSFP28 on the new switches are even better bandwidth/$ - 100Gb for the price of a good lunch :-)
Hope that helps (kudos button is there if it does)
Cheers
Ian