Does Megan Mullally (Karen) Really Have Such a Squeaky Voice?
No, it's entirely forced. Refer to the Pilot episode to hear her real voice. Throughout seasons one and two she slowly transitioned to the more comedic, squeaky voice which wasn't realized during the development of the series.
We never find out exactly. Throughout the beginning of the series, it was comically implied that Karen could be incredibly old. In one episode she accidentally lets it slip that she lived through the Great Depression, but due to the presumably millions of dollars she's spent on plastic surgery she looks much younger. But in the middle of the series, Karen's mother, Lois, comes into the picture. Lois works as a barmaid and can't afford excessive plastic surgery. Lois appears to be in her late sixties.
Were Will and Jack Ever a Couple?
No, never. In a flash back in episode 3.8 "Lows in the Mid-Eighties" a year after Jack and Will first met, Jack confesses he's in love with him. But Will immediately lets him down easy and Jack (obviously lying) says he was just "testing him." Later in episode 5.24 "24" Will and Jack awake naked in bed together after a night of drinking and since they have no memory of what happened, it appears they've slept together and the season ends with a cliffhanger. In the first episode of the following season, Karen reveals information via surveillance cameras that nothing happened between them.