If the person has only 18000 though (300 at 60) then if they put in 100 at $100, they'll only be able to buy 133 at 60.

I would still enter two bids; it's just a matter of adjusting how many at each level.

No, this is not how it works.

All the shares in the auction are sold at the same price. If the market clears at 60, a person entering the bids I described would buy 300 shares at 60.

I apologize if I misunderstood. Isn't this a direct listing? At what point do retail investors get to participate in an auction?

Retail investors (and any other investors) submit orders before the new security begins to trade. The first thing that happens on open is an auction.