hello gnadeau!
Ciao gnadeau,
From the airport you shouldn't have too many problems to get to Impruneta. I suggest avoid entering Florence, head toward the A1 nord exit (from the airport head to the right at the stoplight) and then take it towards Rome... just 2 exits later, exit at "Firenze Impruneta" and from there follow signs to Impruneta.
As far as Casa Morgnana B&B's location and your timeshare nearby, it looks very easy from Impruneta actually. As long as you have the right address for the timeshare, you shouldn't have problems

Those are country roads, a bit narrow but without a lot of traffic so you can go slowly.... the hard part is that numbers are not always clearly marked, so make sure to get good directions from the timeshare, particularly in terms of parking.... if you're supposed to drive up to the gate, you need to know which one because those roads don't permit easy turns back the way you came.
If you're fine with driving those country lanes afterward, the best way to get into Florence is to head toward Pozzolatico and from there up toward Poggio Imperiale which is right behind Piazzale Michelangelo. You can park there or on the streets nearby and from there walk down into downtown or catch the bus 12 downhill, get off along the river right after you've crossed it or right before you cross the bridge.
From your timeshare, definitely stick to the local roads - you're in Chianti so will want to see all the scenery and landscape. The autostrade all go around and are ideal for getting to further places faster of course... but as long as you're in the area, stick to the country roads. You'll see wineries, restaurants right along these roads, just pull off to enjoy!
We'll love hearing how everything turns out! in the meantime, if you need help in anything else, feel free to post!