I'd be risk averse if trying to mix pleasure and business across multiple countries in Europe, so I'd want to be in Venice on Oct 18 and Madrid on Oct 28 if flying in. But you may feel you don't need to be that conservative for Venice since your first commitment is in the evening.
Monday is a day when museums are often closed (Vatican Museums and some other sights are closed on Sundays), so keep that in mind when laying out your itinerary.
I'd opt for visiting Verona on a day-trip from Venice to avoid one hotel change. It's just an hour by train. However, I'm sure hotels are substantially cheaper in Verona, and being in Verona likely gives you a quicker train ride to Rome (or Florence, if you prefer)--though that depends on when you depart. So which do you prefer: fewer hotel-changing hassles or less train time?
I don't know how much of your time in Venice will be taken up with business meetings. That would affect how many days I'd want in the city.
If you want to see some of the smaller towns you've mentioned (all lovely, of course), I think you need to cut either Florence or Rome to allow enough time in your base cities. I'd choose between Florence and Rome based on whether I had a passion for ancient ruins vs. Renaissance art, and whether I preferred smaller vs. larger cities. I suppose it might be a bit warmer in Rome.
Your trip(s) within Italy will be hassle free and fairly quick since there's fast-train service. Getting from Paris to Italy and from Italy to Madrid will be more complicated as already pointed out, so you're losing a good bit of travel time on this trip. I'd figure Orvieto, Segovia and Toledo are each full-day trips, days which can't really be counted as sightseeing time in Rome or Madrid. I haven't been to Siena and Lucca recently enough to be sure about them.
I'd suggest something like this:
Fri, Oct 14: Arrive Paris (4 nights; only 3 good days due to 10/14 jet lag)
Tue, Oct 18: Fly to Venice (4 nights, or 5 if a lot of work commitments; day in Verona)
Sat, Oct 22 (or Sun, Oct 23): Train to Rome (4 or 5 nights; day to Orvieto if you can tear yourselves away from Rome)
Thu, Oct 27: Fly to Madrid (4 nights; one day to Toledo, one day to Segovia)
Mon, Oct 31: Fly home from Madrid
Hey, I've pretty much reproduced your Option 3!
If you feel both Florence and Rome are musts, I'd keep Venice to 4 nights, trim Madrid to 3 (dropping one of the day-trips), and split the resultant 6 nights between the Florence and Rome based on your interests and what impact, if any, Sunday/Monday museums closings will have on your plans.