The speaker claims he is invulnerable because his life depends entirely on the beloved's love; infidelity cannot harm what ceases to exist upon betrayal.
The beloved is his property for life's duration, but the duration is conditional—'for term' can mean 'as long as.' Both possession and temporal limit.
Paradoxical joy in death; happiness and death become identical, even desirable. The couplet undermines this by implying happiness is mere ignorance.
Sonnet 92 resolves the speaker's vulnerability through a paradox: if life depends on the beloved's love, then death comes with betrayal, annihilating the capacity to suffer betrayal. The speaker achieves invulnerability through total dependence—a logical sword-knot. This is darkly comic: the beloved can't torture him with infidelity because he'll simply cease to exist. The arrangement is 'happy' (line 11) precisely because it eliminates agency and extends survival to death itself.
The final couplet shatters the sonnet's constructed security: 'Thou mayst be false, and yet I know it not.' The entire logic depends on ignorance. Happiness exists only in not knowing. If the speaker discovered infidelity, the entire framework collapses. He's achieved perfect vulnerability disguised as perfect safety—he remains invulnerable only by never discovering the truth. Blissful delusion is the cost of his impossible security.
You tell yourself that if your partner ever cheats, you'll just leave—so you can't be truly hurt while you don't know about it. Your happiness depends on plausible deniability. The illusion of invincibility works as long as you never actually find out.