Is it permissible for someone suffering from intrusive thoughts (waswasi) to adopt the easiest opinion, such as the Hanafis' view that the final tashahhud is merely recommended, even if he is convinced that the final tashahhud is a pillar?

The layperson imitates the scholars he trusts. If he is afflicted with whisperings (waswas), and the scholar he trusts issues a fatwa with the stricter opinion, then he is permitted to adopt the opinion of another scholar who makes it easy in the matter. This is not considered blameworthy seeking of concessions (rukhsah madhmumah). The most effective cure for whisperings is to turn away from them and ignore them. Whoever does this will have no need to seek concessions based on some opinions of the jurists.