You're not trusting the miners, you're trusting a system that was designed so that one would need 51% of the mining power to perform an attack. While there is a worrisome level of centralization in mining, you are also trusting the economic incentive structure of the miners is sufficient for no one to coordinate a 51% attack given the current dynamics.
Mining becomes even more centralized due to high cost.
Next, attackers (mean, naughty miners) simultaneously launch / initiate / pay for a DDOS attack on the others such that they can reach this 51% figure?
kind of. But the miners have a financial incentive to follow the system. The system is built in such a way that it's in their best interest to follow the system (except in a coordinated 51% attack, then the system falls apart).
Which is the same thing as saying that you are trusting that miners will be motivated by the financial incentive.
That's a generally sane reason to trust the miners, but the truth is, the mining nodes are controlled by people, and people can be motivated by many things other than money, including fear of people with much political power.