That's a lot for 10 days, but it's do able, depends on the timing of everything and your flights/etc. Especially if you don't spend too much time in Jaco. What time of year are you traveling? (I just read United is added direct flights to San Jose starting in April 2013)
Manuel Antonio park is probably closed Mondays and Tuesdays, all national parks were closed on Mondays for years, but they recently added Tuesday and I'm not sure if that's for every park or most parks.
For the San Jose bus (from Quepos) it is a little bit of a walk to Jaco downtown from where they drop you off. Nothing horrible, but if you've got luggage and stuff you might want to grab a taxi if there's one around. If I recall correctly you might want to make sure you tell the driver Jaco, otherwise they have no reason to stop.
Montezuma is awesome and it's an easy trip from Jaco. Some of the transport companies (but I don't think all) will transport you to where the water taxi is (next town north) as part of your ticket price (I think Montezuma Expeditions does) so ask around before you buy your ticket.
I have to agree with Dave. Best of both worlds with those two destinations, and you won't be missing that much in Jaco, but will be able to have more quality time in your two other destinations. If you are trying to get to the taxi boat to Montezuma, that leaves from Playa Herradura and a van ride from MA is around $85 to get to the boat taxi. Enjoy!