Rainier is supposedly NOT a suitable cherry variety for zone 10.

Zone 10b doesn't get anywhere near 500 hours, more like 75-150.

I have a Rainier cherry in zone 10b and it is packed with blossoms. Just got it last year though so it's too early to tell if those blossoms are going to be able to turn into fruit.

We had a colder than usual Winter though and the tree is in partial shade. I have a theory that being shaded from the sun in Winter can help give the trees more effective chill hours, because in reality "chill hours" do not actually have to do with how cold it gets but rather how long the tree goes without experiencing a prolonged duration of warmth. In other words, even if it is freezing cold for an entire month, just a couple of warm days interspaced in the middle of that is really going to detract from the effective number of "chill hours" the tree experiences. So to try to counteract inadequate chill hours, you don't have to make it cold, you just have to try to keep the tree from getting warm.

In zone 10, a cherry variety like Lapins is going to be on the borderline. It might produce some fruit, but it will be off and on different years, will not produce consistently. I suspect the fact that Lapins is one of the very few self-fertile varieties of sweet cherry helps, because some of the issues surrounding lack of fruiting in warmer climates may actually have to do with pollination.

Right now the only varieties that are considered suitable for zone 10 are the pair Royal Lee and Royal Minnie Lee (you need both for pollination). These were bred in Modesto, California and have only become available within the last 20 years. Before that, zone 10 was not seen as a suitable climate for cherries.