Why pretend to lose, throw away what he had built up to then, and try an entirely different approach to gaining power?

Tentative explanation: he was hedging his bets. If it's a trap, to walk into it would be stupid. If it's genuine, to ignore a warning like that would be stupid, too. He acted in a way that accommodated either possibility.

I think the ritual he performed that night was copying himself into Harry (note to self: this may or may not be the same thing as horcruxing), and the resonance between their magics is a side effect of that. As to which explanation is more obvious, well, I don't think an argument from obviousness is valid in the face of a genuine disagreement, so I withdraw mine. It's reasonable, though.