Will heat help break up the scarring formed because of hematomas after my facelift?

Warm compresses may make you feel better but they should be warm and not hot to avoid injury to the freshly operated skin.  Whether the warm compresses will speed the resolution of the hematomas is questionable but I don't think they will hurt.  The best way forward is to be patient ( as it may take a few months ) and do gentle massage to the area to help break up the residual hematoma and soften the scar area which may occur there.  I personally don't use cortisone injections as there is potential for thinning of the skin in the area of the injection.

First of all there is no evidence that he of any kind will help scars break up. The other issue is proper management of hematoma. If it's evacuated probably it should result in increased short-term bruising but not any significant scarring.

Warm soaks certainly may make you more comfortable and may help and will not hurt hematomas beneath the skin after facelift.  In my experience once the hematomas have solidified only massage and time will make a difference.  Fortunately with massage in time most lumps or bumps caused by small hematomas will resolve but this may take many months.