She also had "a direct line to God", which we do not. Likely God told her that no one that attended to her would contract tuberculosis. I would not use that as an example to the common person that they should go to mass in a crowded church when they are sick. I can't picture too many people being present around her as she was living in a convent.

I did not use her as an example. I merely answered your questions.
She had the same line to God as all Catholics have, but she understood how to use it well, which most of us don't. She wasn't a recipient of special messages from God. 

Right.  I think it's a mix.  People who are scrupulous will seize on the fact that it's a mortal sin to miss Mass ... and they can torture themselves about whether they're sick enough to justify doing something that would otherwise be a mortal sin.  But even the least contagious illness justifies missing Mass ... since charity would require that we not pass it on to others.  Then there are others who go out of pride because it makes them feel heroic ... at the expense of their neighbor who ends up taking ill.  So most of the reasons that would motivate sick people to attend Mass are actually spiritual disorders ... scrupulosity and pride.

Ah yes. The scrupulous Trad.

Which part of "sickness excuses attendance at Mass" don't they understand?  Do they think they're being lax if they let themselves stay at home when sick?

How about accepting the sacrifices GOD'S PROVIDENCE picks for you, rather than your own sacrifices, full of one's own will and preferences? No one chooses to get sick, so at least you know it wasn't your choice. Any truly holy person would jump at the chance to downplay their will and embrace God's will.

Anyone sick and presuming to go to Mass is A) uncharitable toward their fellow Catholics, B) a baby in the spiritual life, and C) probably scrupulous and/or proud. Missing Mass due to illness is a bit embarrassing, and apparently they can't handle it.

Do they think they're being lax if they let themselves stay at home when sick?

Why yes.  That's almost the very definition of scrupulosity, this constant fear of being lax or even just less than 100% perfect.  And, as you noted, it's tied to pride; they are not satisfied if they don't live up to their own idea of perfection.  It also speaks to a distorted view of God, where they see God as waiting to pounce out from behind any bush to zap them if they are not on their guard and being perfect at all times.  I struggled for some years with scrupulosity, so I know whereof I speak.  I was almost instantly cured of it by my confessor at the seminary.  He commanded me under obedience never to confess a sin as mortal unless I could basically swear to him before God that it was a mortal sin.  He told me that if I were wrong about it, then it would be on him, and that God would not judge me for it.  So I obeyed ... and was immediately at peace and cured.  Now, some people are SO scrupulous that they would just write off the confessor as wrong and assert that they know better.