Root canal sanity check

I got a pretty deep filling on tooth 31 6/3 and have had cold sensitivity (lasting 10-15 seconds) since. Went to an endo and they said I need a root canal and crown lengthening, I spoke with my normal dentist and they said that is normal but I am still a bit confused about the need for crown lengthening and if a root canal is my only option at this point. I did tell my dentist I am extremely busy with moving and work trips during August so I really need to be sure this needs to be taken care of in July or mid September and she said I should do July.