"a soul can never be forced upon the unwilling, you were never in any danger from me" is, I think, emblematic of Mythal in a really juicy fucked up way.
We know that possession in DA requires a 'yes' in (most) circumstances. But we also know that that consent can be coerced; coerced consent is actually how 99% of demons operate.
It's probably true that Morrigan would have to say yes. But the way in which Flemeth raised her was designed to give her absolutely no choice in the matter. We see, consistently, that Flemeth emotionally abused Morrigan over and over. She kept her purposefully isolated from her peers, consistently needled and put her down, and with Morrigans mirror literally shattered a symbol of Morrigans individuality. Flemeth even has special robes for Morrigan (robes of possession) which reduce her willpower. In the fade, Morrigan says the spirit pretending to be Flemeth is acting more like her mother only when it physically slaps Morrigan suggesting a level of physical abuse.
In DAI, just before she says the famous 'you were never in any danger from me' line... Flemeth literally gives a horrid little ultimatum to Morrigan that proves just how manipulative she is. She literally says; I will take your son from you forever OR you can have him back but you'll never be safe from me as long as you live. And Morrigans response 'i will not be the mother you were to me' also points at the abusive Morrigan suffered under Flemeth.
The way in which Morrigan was raised then, was designed to create a person who would say yes, who had been groomed and emotionally (perhaps even physically) manipulated all her life for this one purpose.
Mythal seems to be characterised in a similar way; she is benevolent, the 'best' of the gods, and she believes herself to be compassionate. And yet, she kept slaves. You can drink from her well but once you do your bound to her will and will do anything she says. She sees herself as the good one, good to the people, but she's still willing to use them.
To reduce Flemeth down to a loving mother who never meant anything bad for Morrigan because of a single line she says (which itself is a manipulation) is to deny a lot of depth to Flemeth, Morrigan and Mythal.
