Getting from Florence to Seville via VCE and MAD
 

Long story short, we must take the 8:30AM train on a Sunday morning from Florence to Venice, which arrives around 10:23AM (earliest and fastest train possible), the earliest AVTO bus (fastest option?) to VCE airport we can catch is probably the 10:45AM one and it's about 20 mins trip so we should arrive at VCE around 11:05-11:10AM. Now, our flight from Venice to Seville via MAD, Spain departs at 11:50AM sharp (Iberia). Again cannot change the VCE-MAD-SVQ flight (all other options are gone). We are flying economy, have Priority Pass.

Are we doomed? Is the new airport much better in terms of moving people through? If we purchase the 40 euro SAVE pass, will that help us?

We have bags to check - but if necessary, I'll just make sure we don't need to check any bags.

My nephew recently spent a month in Florence and last week took that exact train to Venice for the day. If every single thing goes right, you will arrive in VCE on time. Given your departure time out of Marco Polo Airport, I suggest taking a water taxi to the airport. Yes, it will cost 5x the price of the bus, but you will definitely arrive in time. I think the scenario you describe is cutting it way too close . The new airport is very nice but I am not impressed that they move people through quickly, with or without Priority Passes.

@OP: Have you thought of going to Venice the day before and spending the night there? You'll be completely safe...

The 07.54 Italo gets into Mestre at 0948 then a 15 minute cab ride to the airport. You'd be there more than 1.5hrs before the flight

I think the smoothest option is to just buy FLR-MAD-SVQ on IB. Economy tickets go for about $100, and you save yourself the hassle of getting from Florence to VCE. It's probably cheaper, too.

I have a hard time thinking that flight is sold out - what's the date of this leg of your trip?

OP, In Venice, you would also have a 5 or 6-minute walk from Santa Lucia train station to Piazzale Roma, over the badly misconceived bridge with the variable-pitched stairs (Ponte della Constituzione) where a misstep is practically unavoidable (I had a few) and you have to lift and carry your luggage.

If you take the train, don't go all the way into Venice (and, of course, don't take the water taxi). As mentioned above, get off at Mestre. You will be a little closer to the airport than from Venice proper, and also earlier by 12 minutes. The ATVO bus is the airport shuttle, just turn right as you exit the train station. Unlike in Venice, the bus is directly at the train station. It runs to a similar schedule.

According to the official time table, the bus from Mestre should take less than from Venice (17 minutes vs 20), but when we took the 7am bus on a Friday, it took almost 30 minutes, making frequent stops for commuters and schoolchildren (yes, it was the ATVO Airport Express, not the regular ACTV local bus). There should be no such problem on a Sunday.

The taxi rank is right at the train station exit, it may be your best bet. It should cost between €40-50.

ETA: As mentioned above, you should take the earlier Italo train (biglietti.italotreno.it). The are no more seats left for tomorrow, seats are available for the other Sundays. You should book in advance (the cheapest tickets are €24.90 pp).

Let us know how it went.

As often happens, people don't provide enough information. A water taxi, depending on time of day, might cost
110 euros. When you say, "us" what do you mean? Water taxi can hold up to 10 people, in which case it's11 euros per person, cheaper than a land bus. It's not clear what you mean when you say, "us." Two of you? Ten of you?