Looks like changing the dates does reveal the Base fare availability on my travel dates. I guess I don't have to worry so much about it selling out just yet. But wow is Base fare cheap!
Also, does splitting my time in Peru 65/35 between Cusco (and MP) / Mancora seem reasonable?
That would depend on how long you're staying... I think that you can see Cusco in 4 days or so and have enough time but it all depends on what you are planning to see... In Mancora you can just relax and maybe try some water sports for as long as you want.
Are you traveling on the same day from Cusco to Máncora or are you staying for a day or so in Lima?