Pax you are correct about the Sacrament of Confession part, of course. That is only open to Catholics.
But the Protestant theoretically could be "justified" by an act of perfect contrition. That is part of normal Catholic doctrine. Not that such would be common. And if attempted, it would be hard to pull off. That is why Jesus gave Catholics the Sacrament of Confession. The Sacrament allows sinners with imperfect contrition to receive absolution. Most of us will only achieve imperfect contrition.
Yes and no. It depends. There are various types of heretics and various types of protestants.
1. A protestant like Charlie who has never been a member of the Church, and never received the sacraments of Eucharist and Confession. Justification could NOT be had by them in the same way as Catholics because Trent's guidelines are two-fold. 1) perfect contrition and 2) vow/promise to confess. Protestants like Charlie can't promise to confess because they've never done it before.
2. A fallen-away catholic turned protestant. Let's say, similar to Martin Luther. Yes, he could repent on his deathbed and become catholic again. Because he was catholic once before, fully.
3. A schismatic orthodox. Again, they have received the sacraments and believe in them. They could convert on their deathbed because all they would have to do is reject their schism.
Someone like Charlie is a 'simple protestant' (never been catholic, except by baptism) has a VERY HIGH hurdle to get into the Church. They have to become members first.
But an apostate, or a catholic heretic, or an orthodox heretic...these are members of the Church in a more tangible way than a 'simple protestant'. 2 totally different categories because a 'simple protestant' has never received any of the major sacraments and is not a member in any way (other than baptism).