Let's try to be clear!
1) There are only two efficient and effectives ways to get from Seville Airport to Faro Airport!
a) Car
b) Bus/Private Transfer
2) Seville Airport to Faro Airport.
According to Google Maps the distance between the two is 213 km or about 2 hours 6 mins.
I live in the Algarve and have driven this route many times, the last time 2 weeks ago, and I would suggest 2hrs. 15mins is nearer the target if traffic in Seville is heavy.
a) By Car
There is one company that does, at times offer, reasonable priced car hire from Spain to Portugal, specifically Faro Airport. I know that some people have encountered difficulties with them in peak season owing to a shortage of cars, but is is worth a shot.
See: http://www.iberiacarhire.com/
b) By Bus
Firstly, you will need to get a the airport bus from Seville Airport into the Prado bus station and then a C4 local bus to the Plaza das Armas bus Station.
By September, as Cubsur says above, the bus frequency between Seville and Faro will probably be down to 3 buses per day. We are unlikely to know the schedules until early September but it is unlikely they will differ much.
Based on last winter, I reckon the applicable daily bus timetables will be as follows :
DAMAS/EVA
Sevilha 07:30 Faro 10:10
Sevilha 16:15 Faro 18:55
ALSA.es
SEVILLE 23:59 FARO 02:15 next day
You will then need to get a local bus from Faro Bus Station to Faro Airport.
Bus times here on Cubsur's site: http://bit.ly/c2NFqI
As regards private transfer:
These guys are currently quoting € 226.50 for transfer from Seville Airport to Faro Airport.
http://bit.ly/9UdwBj
I am sure that other transfer companies, such as polotur or algarve-365.com will quote similiar prices.
I really do not think dropping of cars in Huelva or Ayamonte will make the trip any quicker or cheaper. Huelva is a pig of a city to drive around and then finding the bus station is not easy either.
Ayamonte is OK. However, you have to get the ferry across to Vila Real de Sta. Antonio, and then either train or bus to Faro and then a bus or cab to the Airport. One other point though, is that, if I remember correctly, several car hire firms offices are not in Ayamonte itself but on the Isla Canela nearby. That means more taxi fares back to Ayamonte.
From my perspective, you are as well dropping off the car at Seville Airport and taking the bus direct from Seville to Faro, even if it means adjusting your plans..
