I played BGDiA with a dwarf cleric, and we continued on to very high level (we stopped at level 18). Assuming you've got a feat to support concentration saves (Resilient or Warcaster), Spirit Guardians is by far the best option. Running into melee then Dodging is a solid choice, because you really don't want to lose the spell and you'll almost certainly become target #1 for attacks. The biggest advantage of SG is that you only have to cast 1 "combat" spell each fight, saving the rest for healing and utility (both of which will be an issue in the adventure). In theory you can also cast Spiritual Weapon, but I found this burns too many spell slots for my taste. Remember to always have 2x level 3 spell slots available at all times for Create Food and Water and Revivify, because you shouldn't need them... until you do.
As for the rules question, IIRC the official ruling is that "holy water" doesn't clarify how much, so you don't have to spend a full vial. Since it doesn't require a full vial, it lacks a cost and can be covered by a focus. However, your DM's ruling is final, so talk with them before deciding not to get some.