By the time the pair were finished, Kapiti were in tatters with seven wickets down for 60-odd, allowing Tech to even consider the precious bonus point by dismissing Kapiti for less than 97, which they did with 10 runs to spare through wickets by Caleb Greene, Jessica Watkin and Chris Friedel - with his first and only delivery of the match.
Opening batsmen Harding and Greene had got Tech off to a big start with pull shots and drives for 22 runs from the game's second over, punishing A Murray with sixes, boundaries and a run three.
However, both of them were bowled by Carter Andrews, setting a trend where most batsmen got out swinging.
Lock (43) looked to steady the ship, but was frustrated as no one could stay with him, with leading hope Kinnerley (28 from 21 balls), giving up a nick behind to Harrison.
The final five wickets folded for 18 runs, with Harrison taking 3-8 from four overs, Andrews 3-19 in his eighth, and Murray making a good comeback from the early shellacking with 3-63 from 10.
But Kinnerley and Harding would just grit their teeth as they trundled in for over after over.
Following Purvis' two catches and the caught behind to dismiss opener Ryan Hickey, the big man Harding would get it straight on the dot, with the fielders and Friedel in particular appealing for everything.
Harding bowled Andrews and got No6 batsman Jason Smith plum for his five-for.
At the other end, Kinnerley was let down slightly by some Kane Watkin misfielding behind the stumps, extras taking his tally to 47 runs from 10 overs, but he still clean-bowled the last of the middle order batsmen despite being very tired.
Only one Kapiti batsman made double figures with 18 not out, as Greene came on and picked up a Harding catch with one of his worst balls - a high full toss - compared to no reward for beating the bat on several occasions.
Watkin induced a mistimed drive into Friedel's waiting hands, and then the veteran came on and with his first delivery a cut shot was snaffled by Lock to make up for an earlier miss.
Lock confirmed that even though they looked exhausted, the chips were on the table by asking Harding and Kinnerley to bowl out from the beginning.
"We had to, we had nothing to lose. They wanted to keep going as well ... We've done all we can do [to finish top 4].
"We came back. We came out, attacked with the ball, did the job."
Harding admitted he had been feeling the effects of the festive season during the uninterrupted spell - just missing a caught-and-bowled chance with tired legs was proof of that.
"We just wanted to bowl them out quickly, so we just ran in quick. It's all about trying to get in the semifinals."
Meanwhile, United fought every step of the way with Weraroa, as the leading batsmen Matt Boswell (30), captain Gerard Hobbs (21) and Greg Smith (22) all got starts but fell when someone needed to crack on, to be 89-4 in the 21st over.
Enter Brendon Walker (77 from 72) for another vital innings, striking eight boundaries and four sixes, carrying the team through until being bowled in the 47th over, running through several batting partners with Martin Pennefather (15) and Ryan Slight (13no) offering the most support.
Other hopes in Chris Sharrock and Robbie Garwood fell to crucial LBW decisions.
At 245 all-out in the final over, United were 20 runs short of being able to put the game out of reach, as Weraroa looked to their standout opener Matthew Good (82 from 76) to shore up an end.
Partners stayed with Good for steady intervals before United managed to hit back in the 26th over with Sharrock getting him caught behind by Boswell for 123-4.
Dylan Martin was very economical, holding the batsmen to just 17 runs from his 10 overs, with three maidens.
Nonetheless, Weraroa were continuing to creep to the promised land through Wade Anderson (57no) and Jason Strickland (27), before United again struck back as Walker and Pennefather hit the stumps to leave the home side 199-7 in the 42nd over.
But the matter was decided by captain Huka Kahu, who came out and peppered the domain's short 40m boundary with 26 from 19 balls, hitting his team home with the first ball of the 49th over.
"Every possible roll went against us," said the disappointed Hobbs.
"They have a couple of short boundaries, it was just one of those games.
"It's a close competition - the team coming first, if we'd won they would have been knocked out.
"Just not our day, it's a hard pill to swallow."
United will now focus on the January 24 clash at New Plymouth's Pukekura Park with the Taranaki one-day champions for the national zonal qualifiers, and Hobbs said they will have key allrounder Tom Lance back from injury as they try to become the first Whanganui team to win the annual fixture.
In Saturday's other games, Paraparaumu CC confirmed their home semifinal with a comfortable five-wicket win over an understrength Bayer Marist in Levin, reaching 111-5 after bowling them out for 107.
Matt Burke Engineering Marton Saracens' season of misery in 50-over cricket continued with a nine-wicket loss to Levin Old Boys, who wrapped up third spot on the table at Donnelly Park.
Marton slumped from 52-3 at drinks to all out for 84, with Levin then comfortable at 85-1.
It is worth noting that in the Whanganui Premier 1 Twenty20 competition, played when the representative cricketers are on Hawke Cup duties, Marton are undefeated and will be eyeing up that trophy with a couple of weekends left to play, including this Saturday, while Active Physio Wanganui are away playing Manawatu at Fitzherbert Park.
