Listen, i think im not making myself clear enough.... replace Zoro with Jinbei from the beginning. The qualities Zoro possess: strength, peudoleadership, the ability to offer Luffy advice... all those qualities Jinbei poessess. And on top of that Jinbei is an amazing helmsman.
I've throwing around the word essential since this discussion started so im going back
Nami is ESSENTIAL because she is a navigator. Luffy cannot become PK without a navigator
Robin is ESSENTIAL because she can decypher Poneglypphs. Luffy cannot become PK without her.
Chopper is ESSENTIAL because of his medical expertise. Luffy cannot become PK without him.
Franky is ESSENTIAL because he fixes the ship. LUffy cannot become PK without a shipwright.
Sanji is ESSENTIAL because he is a world class chef who knows the dangers of eating mysterious food and can now prepare energy food. Luffy cannot become PK without a chef.
Ussop is ESSENTIAL because of his ability to look at things from a distance, combat and noncombat (and was a temporary shipwright before Franky). Luffy cannit become PK without a Sniper.
Brook is NON-ESSENTIAL because he isn't the best combatant (can be useful) and is a musician. Luffy considers a musician mandatory for his crew, but Brook isnt NEEDED for Luffy to become PK.
Now...
Zoro is NON-ESSENTIAL because all he is on the crew is a combatant. He doesnt have a dual role. He is the first mate, but like you said Jinbei and Sanji have taken the role at different times. He offers advice to Luffy, but so do the others when times call for it. Zoro ISN'T NEEDED for Luffy to become PK. We have 2 others on the crew that can easily fill his role.
Now there was confusion and M3J thought i was saying Zoro is useless. That is NOT what im saying. Zoro is 100% useful, his feat on Thriller Bark speaks to that. He is courageous above everyone in the crew and is loyal to a fault. He is a great friend and ally and god have mercy on your soul if you harm his nakama and make them cry.... but he isn't needed on a crew for the captain to be PK.
Hell, Jinbei also sacrificed his life to protect Luffy from Akainu burning a hole through him
Jinbei is every bit as courageous as Zoro and just as willing to sacrifice his life to his captain, he has a triple role as helmsman, combatant, and advice giver to Luffy.
Zoro can be both useful AND non-essential. The 2 arent mutually exclusive.