There are few reasons fares are so much higher further out.
Like GM said, the green season may not be the peak season for traveling to CR but it is the peak season for traveling everywhere else in the northern hemisphere. The demands for equipment (jets) and jetfuel SYSTEMWIDE is much higher and that also puts upward pressure on the price of jetfuel which effect ALL flights. By October that demand may be back down but the predictability of operating costs, particularly fuel prices, that far out is still much more uncertain. Why would airlines, lock themselves into fares based on TODAY'S costs when the costs at the time you fly might be so much higher.
More importantly, futures airfares are so much higher for a far simpler and basic reason. Why discount fares way so far out in advance when you still have so much time to sell at least some of those seats at full fare (or at least without discounting it so heavily)? I'm absolutely certain you'll find as the time gets much closer to the actual date of the flight that the unsold seats will become available at some discount as well. How much of a discount will depend on the state of the market at THAT point in time. Usually the real discounting extends around 3 months out. There may be even more heavy discounting that takes place as you get even closer to the departure date if lots of seats remain unsold.
However, usually at some point really really close to departure, the fare shoots way back up. The reason for that is that, vacationers (who are the real bargain hunters and who have some choice over the exact dates they travel) usually book way in advance. Business travelers often don't know when they'll have to be taking a trip until very shortly before and are then willing (or forced) to pay whatever price the airlines ask. This is also the reason for the "Saturday night stay" restriction, since, unlike vacationers, business travellers usually want to get back to their families over the weekend. Standby fares and other last minute fares are the absolutely cheapest since at that point the airlines would rather get something for the seat then see it go empty and make nothing at all, however the availability of those seats are never guaranteed and thus don't usually work for most business travellers (or even most vacationers).
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As for those guaranteed fares, it all depends on what sort of agency or website you're talking about. If its some sort of consolidator, then you'd have to check with them whether the air miles are transferable. If its a booking site such as Travelocity or Expedia, you'll have to check the restrictions on both the specific ticket you're booking and for that website itself. Airfares come with all sorts of restrictions - refundability, minimum stay, maximum stay, Saturday night stay, change fees, blackout dates and POSSIBILY being not valid for miles credit, though I don't recall ever seeing that for any airline I've flown. Whatever rights and restrictions that apply from the airlines will extend to tickets you book through one of their agents like Orbitz, Expedia or Jose's Travel Agency down the street. Those price guarantee programs also come with MANY restrictions, though, again, I've never seen denial of miles credits as being one of them.
For example, reading the small print at Expedia:
1) You have to find the lower fare within 24 hours of booking your ticket.
2) You have to notify them within 24 hours of booking your ticket.
3) That lower fare has to be available at the time you notify them as determined by the CSR.
4) That lower fare has to be for the EXACT same itinerary - dates, times, stops, carrier and even class of service (naturally you can't use finding a lower coach fare as basis for claiming a refund on a 1st class ticket).
5) That lower fare has to be available to the general public ON-LINE, ie it can't be available as part of your corporate discount program, membership club, rewards program, use of special coupons, phone number you have to call, through a consolidator, etc.
6) The fare difference has to be more than $7.
7) IF all of that checks out they'll refund the difference to your credit card but if you booked an air/hotel package and want to get their $50 credit guarantee you'll still have to send in adequate documentation within 30 days even AFTER calling the customer service number and then they'll send you a COUPON good for another Expedia purchase that expires within 1 year if you don't use it.
As you can see, there is nothing in there about not getting air miles, but there is plenty else in there to slip you up and make it difficult for you to get back the difference.
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