Two questions:

1. Do the attackers (users who abuse this), modify all transactions they see or do they target those transactions that only involve their wallet?

2. Even given the relative ease an attacker would have with modifying a transaction, that transaction still needs to be accepted by the majority otherwise their modification is ignored. So given 2 transactions for the same bitcoin transfer, I would guess that 50% of the time their transaction is accepted. I guess that the attacker could modify the transaction, for example, 4 times and then it's an 80% chance that one of their modification is accepted (4 out of 5). Do I have that right?

This was a good article, I enjoyed it's breakdown with examples.