Boeing 737 successor Part II

A few days ago I wrote about a possible 737 successor, aka 797.
Every day I get a mail with the headlines from ATI (Air Transport Intelligence) and today there where two headlines that catched my eyes.

1. Boeing sets performance targets for 737 clean-sheet contender (10Mar11
   23:29 GMT) Boeing would like its possible 737 clean-sheet design to
   have a 15-20% improvement in fuel efficiency over today's
   model, VP for marketing Randy Tinseth says at the Asian
   Aerospace show in Hong Kong.
2. Boeing: Twin-aisle 737 replacement could increase aircraft utilisation
   (10Mar11 23:25 GMT) Boeing is evaluating how to reduce weight from a 737
   replacement with a wider fuselage accommodating two aisles,
   vice-president for marketing Randy Tinseth says at the Asian
   Aerospace airshow in Hong Kong.

Here are my questions (and in the unlikely case that Randy Tinseth reads this, I would be the most happy man to get the answers directly from himself):

1. Why does Boeing only target 15-20% better fuel efficiency over the 737? If this is all they want, they can go straight ahead with reengining (Mike Bair said that reengining would yield 11%) plus some (more or less) minor improvements like weight savings through more composites and aerodynamic improvements. As a reengining benefits from grandfather-rights, Boeing would not have to care for some safety regulations that were introduced after the EIS of the 737-100. If Boeing goes with an all-new aircraft, they would have to care for all these with a weight disadvantage for the new aircraft versus the B737NG from the beginning.
2. Aside from the problems I tried to describe in the earlier entry, I still do not understand how the twin-aisle concept reduces the time for passengers to board and de-board an aircraft. The bottleneck are always the doors, so Boeing should better build wider doors. And for some low-cost airlines they should maybe have an option to build in a same-sized rear exit door, as Ryanair usually does not use airbridges at the terminal but boards and deboards on the tarmac via stairs.

Looking forward for the Paris Airshow this year more than ever...

There are three more parts of this story:
Part I
Part III
Part IV