We did the reverse drive a couple weeks ago, but we did not stop at Poas because it is supposed to be best in the morning (less cloud cover) Since we left Manuel Antonio in the morning we couldn't get there early enough. The drive took us 6 hours, and we stopped only to use the facilities and for about 30 minutes at a roadside stand to drink a Pipa Fria.

I think the reverse drive that you are making will be much easier and I wouldn't recommend anyone do what we did. We ran into heavy fog on the way to Arenal in the early afternoon and it was a very difficult, frightening drive. The fog is usually lighter in the morning so you should be alright, although we did encounter pretty foggy conditions when we left Arenal at 10 am on our return to Alejuela. It may have been worse because it had been raining for at least two weeks straight (in the dry season!)

It will depend on the route you take; the logical way on the map results in some rough construction (4WD) unless they've finished repaving the damaged road between San Miquel and Peace Lodge (126). It's a nice drive: here's a map to go from Fortuna to Peace Lodge (then it's very easy to Poas) http://www.waterfallgardens.com/pdfs/from-arenal-la-fortuna-through-san-miguel-to-peace-lodge.pdf
(Fortuna--Muelle--Aguas Zarcas--San Miguel--Vara Blanca to Posito then to the park.) You really need to be in the park very early to beat the cloud cover, so EARLY departure would be necessary. If it is cloudy, give it at least 15 minutes; the clouds roll through and visibility can be terrific for a minute or so (worth it).
From Poasito to Alajuela is less than 1 hour, then you can hit the main (new) highway and make it to Quepos in ~3 hours (don't forget to stop at Tarcoles for the crocodiles).