The goofy idea that the parma magica acts like some kind of force field is not consistent with the information that we have about Magic Resistance. There are a number of examples in the rules to define how it works:
A normal rock continuously propelled by magic stops at her skin or clothes. The maga feels
the warning that something has been resisted, and also feels the rock touching her (before it
falls), but she is not struck by the rock. It is as if the rock had been moved up against her as
slowly and carefully as possible.
The maga can feel the rock; it touches her. It simply does not harm her.
If the maga steps onto a magical bridge, it remains and will bear her weight.
The weight of the maga is ON the bridge: she is touching it.
If the maga falls onto a magical bridge, she is stopped by the bridge, and takes falling
damage.
The magical bridge causes the maga damage, despite its magical nature: she obviously touches it if it damages her.
If the magical bridge falls onto the maga, it is stopped, but does her no damage.
Whether the magical bridge affects the maga is entirely dependent upon the outcome to be expected. There is no force field always shielding her from any sort of touch -- the magic's effect is entirely situational, and changes with the circumstances.
If a maga falls into a pit full of magical spikes, she either slides around the spikes if there is
room, or finishes lying on top of them if there is not. She takes normal falling damage, but no
damage from the spikes.
Note that here it's the maga being affected and moved around by her own Magic Resistance.
An invisible character cannot touch or exert physical force on the maga, even if the
invisibility spell had Personal Range. The maga feels the warning that something has been
resisted, but feels nothing else. See the magical bridge examples, earlier, for further
discussion.
I don't find it particularly helpful to know that I can walk on an invisible person, fall on an invisible person and be hurt by that, but if they happen to fall on me I'll be fine, but this is what we've got. Per the "normal rock continuously propelled by magic" example I'd say we're to assume that the maga feels the invisible person touching her but that the invisible person is stopped from exerting physical force on her.