Hydroguard will help for sure, just figured you can lower the actual temp with that simple idea.
As to your 2 strains recovering and growing at different rates. I would say one is just more prone to stress than the other and is more likely to be slower to recover. Everything we do to them takes attention away from new growth. Some just are quick to heal and refocus on growing more than others.
Looks like the FP roots are slightly behind the other, the WW cross looks like it has more roots coming off roots, side roots, I'm sure theres a name for them but you get what i mean. Could be just strain difference but try as much as you can to make sure PH conditions are identical.