Is it correct that multiple Traumatic wound effects stack? So if you receive 2 Tr level wounds, you are now at -8D?
Yes. All wounds stack.
And if/when these 2 Tr wounds bleed out to 2 Mo wounds, you would need to spend 2 artha to invoke Will to Live?
p496 looks like it could be read both ways, but my read would be that paying for the Will to Live protects you from any and all Mortal Wounds you happen to be suffering.
I think Will to Live is more there to be a check for the players – “Do you want/dare to keep playing this character?” – than an actual resource killer. (That comes from recovery and lifestyle maintenance!) So one Persona point to survive all your Mortal wounds is fine by me.
This fits the let it ride concept. You invoke the will to live and let it ride for that scene.
The recovery isn’t going to be any easier though.
To pile on: with this interpretation, spending your last Persona point makes a strong statement. You’re all in.
The other way, you can be all in without realising it.