What are the symptoms of an alternator with "weak diodes" on a late-model Volvo? Will a multitester still detect output of 14.4-14.7 volts?

DETAILS: For the past week, my 2009 S40 2.4i has experienced a slow cranking early in the morning. It seems as if the battery is almost drained. One morning, I had to try 2-3 times. The engine finally cranked, idled roughly for 30-60 seconds, and then the idle smoothed out. Once started, I usually drive 20-30 minutes in mixed city/hwy traffic with no problems. When I get to my destination, the vehicle can restart with no problem. The car usually sits 6-8 hours in a parking lot at work. In the afternoon, there is a slight hesitation (1/2-second) but it then cranks up and seems to run fine. Fuel economy dropped from a 6-month average of 28mpg to less than 19mpg this week. NOTE: I have cleaned battery terminals and several ground points. Same behavior all week.

I took the vehicle to a local auto parts store (O'Reilley's) for a quick battery test. They said the battery is fine and that the alternator is gradually failing due to "weak diodes".

However, I connected a multitester several times and found:
- Battery, car off: 10.8v
- Battery, car on, idle, no accessories: 14.4v
- Battery, car on, idle + accessories (lights/AC/radio): 14.4v
- Alternator positive post, car on, idle, no accessories: 14.4-14.7v
- Alternator positive post, car on, idle + accessories: 14.4-14.7v

From my research and past experience, a failing alternator should produce much lower voltage. Furthermore, the battery would have drained completely after a week if the alternator was failing to recharge it. Is my reasoning correct? Am I missing something about alternator performance? Or should I demand that the parts store replace my 6-month old battery?

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