Hi firewns,
This was one aspect I had questions about and discussed this with Lal previously. Most of what I am writing is based on what I learnt and understood from those discussions.
With regard to your question, of whether taking off life support of a parent leads to an ananthariya kamma; If someone commits these kamma, whether knowing or un-knowing, then he has committed an anantariya kamma.
How would sotapannas and other ariyas handle such situations: A sotapanna will definitely not do those 5 anantariya kamma. Not everyone has or will face such a situation. This may mean that a sotapanna will not face a situation to do such a kamma.
So the best method should be to avoid such situations. May be leave it up to the doctor, to decide. Also the financial side comes to play. But if you believe that one will go to the niraya because of this act, will you really bother about the finances? Can any amount of money come to your rescue in hell?
With regard to putting an end to life due to suffering; the worst possible suffering anyone can undergo as a human is not even comparable to what one will have to go through in the niraya. So ending a life to put an end to suffering does not make sense.
If one has commited one of these acts then there is no way to eliminate the vipaka from it. But yes they may be able to reduce the extent of suffering. This can be done through doing kusala kamma as much as possible and kusala kamma which have bigger impacts.
Also someone who has committed such an act will not be able to understand the dhamma and become a sotapanna. That is why the Buddha went to angulimala before he killed his mother, because if he did kill her, he would not have been able to understand the dhamma.
In the Samannaphala Suta https://suttacentral.net/dn2/en/bodhi it is stated “Soon after King Ajātasattu had left, the Exalted One addressed the bhikkhus: “This king, bhikkhus, has ruined himself; he has injured himself. Bhikkhus, if this king had not taken the life of his father, a righteous man and a righteous king, then in this very seat there would have arisen in him the dust-free, stainless eye of Dhamma.” So this shows that someone who has committed an anantariya kamma will not be able to understand the Dhamma, and become a sotapanna.