The game proved an effective blooding for the six newcomers to Heartland rugby – starting prop Tai Pulemagafa and reserves Keightley Watson, Slade Hay-Horton, Regan Collier, and try-scorers Kahl Elers-Green and Tiari Mumby.
Whanganui's other try-scorers were second-five Timoci Seruwalu, hooker Roman Tutauha, No8 Semi Vodosese and winger Josaia Bogileka.
They got good service from the kickers, with fullback Ezra Malo slotting four from six, while reserve first-five Ethan Robinson came on to get the last two conversions.
"We had a plan of what we wanted to do, to start with," said Hamlin.
"At 24-0 up, pretty much all we wanted, but then it's all the things that haunt us.
"Just a dropped ball, a missed tackle, and we were under pressure and went back to 'club stuff'. We played with a man in the bin for ten.
"While it's a hard lesson, it's a good lesson for these boys to learn.
"Slip off the mark a bit [and that happens]. Now they really, really know it.
For his first game in the hot seat, Hamlin naturally wondered if his preparations had all gone right at 29-19 with less than 20 minutes left and West Coast driving forward, but he need not have been concerned as Whanganui absorbed the pressure and then "pushed the envelope", with their discipline at the break down improving.
"The people that came on at different positions, they all did their job.
"We were able to punish them."
The coach singled out blindside flanker Ben Whale for an inspiring 80-minute performance, while playing alongside three debuting props, young veteran Gabriel Hakaraia stepped up to go around 72 minutes in the front row furnace.
Hamlin also wanted to single out lock Matt Ashworth and Pulemagafa, the latter not named in the original squad, who he hoped had silenced any whispering critics about their inclusion.
"Tai didn't look out of place. He showed today he belongs in this arena."
While he has taken the captaincy reins a few times before, the game marked first-five Dane Whale's ascent as the incumbent skipper, and he had to laugh how his first test of leadership was the weather forecast being completely wrong.
"We talked about it all week – wet, wet, wet.
"Coming down - wet. In the morning - wet.
"Then at lunchtime it cleared and hot-as. That's tailor-made for us, but we wanted to play a territory game anyway.
"Just a few momentum shifts. [West Coast] kept climbing and key carrying.
"Proud of the boys to pull it around. Glad the bench did their job."
Knowing how important a first up win can be with only eight games to qualify for the Top 4, Whale was pleased all the debutants got the sense of just how unique and tough this competition can be.
"Proud of what they added and got to get them on the ball – because that was a classic Heartland game.
"[It's important] once you know what it's like, so you can keep building on that."
Whanganui 43 (T Seruwalu, R Tutauha, S Vodesese, J Bogileka, T Mumby, K Elers-Green tries; E Malo pen, 3 con, E Robinson 2 con) bt West Coast 19 (E Smith, T Tauwhare, J Tomlinson tries; B Wyness 2 con). HT: 24-19.
