Freeza stated he was holding back and I believe Vegeta and Goku also had similar statements.
As for fusion, it isn't anywhere as big of a boost as you might think. I'll break it down for you:
- SS Kale is SSB-level while SS2 Caulifla is just SS-level
- SSR Black is SSB-level while Zamasu scales to SS-level in the manga
- Merged Zamasu is equal to CSSB Goku and two copies of himself are vaporised by SSB Vegeta's Gamma Burst Flash.
- Earlier in the manga it's stated SSB becomes 1/10th of its power after being used beforehand, meaning CSSB Goku is only 10x stronger than SSB Goku
- So a fusion of an SSB-level fighter and a SS-level fighter is around a 10x boost at most
- So Kefla is at most 10x stronger than Kale
From there we can't really scale how much Goku, Vegeta and Freeza were holding back relative to Kale but it's worth noting that both Freeza and Vegeta think they could deal with her if they went full power.
Would that still make Gohan and Kefla significantly stronger? Probably but it would be a <10x gap in power.