I'll be blunt. No matter what you've learned from Singing Success, mixed voice doesn't exist. There are 4 scientifically validated vocal registers (notice there is no "mixed register".)
m0 = vocal fry
m1 = chest register
m2 = head register
m3 = whistle register
In all the years of studying the voice from classical tenors, sopranos, and even countertenors.. no evidence of a "mixed register" has ever surfaced.
The chest register (m1) can SOUND like the head register and the head register (m2) can SOUND like the chest register (through resonance & vocal cord tension strategies.) But there is mysterious "mixed voice."
Similar to my "chest voice vs. head voice" article, the problem with mixed voice is mixed up definitions. Some people (primarily women) will say mixed voice is a "bright, nasal head register (m2)", others (operatic tenors perhaps) will say mixed voice is "mixed resonance within the chest voice (m1)", and classic rock vocalists will say mixed voice is "blending chest (m1) and head (m2) to sound like each other."
Unless definitions are clearly and objectively defined before opening our mouths about mixed voice, the discussion is absolutely pointless.
Keep it simple. Stick to the science.