I just got back from Cabo.
I went through a travel agent who booked through Delta vacations. This gave my guests the option to put down $150 deposit and pay off the balance 30 days prior to departure. I just went with a loca travel agent. You can look around your city and see if anyone is offering any specials. With Delta vacation, they gave you a rebate of $25 per room that was booked. I also know few travel agencies that offer free package for you if so many rooms are booked.
I don't think it's too far in advance to reserve your wedding date.
I sent out my STD as soon as we reserved our date and finalized the plans. For a destination wedding, I don't think you can be too early. This gives people time to review their vacation time from work and finances. I just reminded our guests that payments had to be paid in full 30 days prior to departure. I did not give them a deadline. However with it being a destination wedding, you pretty much know who will be attending or not pretty far in advance. I had couple people show up at the last minute but one was a friend from Dubai who surprised me.
I got married at the Riu Palace and the WC there didn't communicate with you much until 2 months prior to the wedding. So, don't worry about the details. You can ask basic questions, but they can create your dream wedding.
I know another bride who is getting married at Riu Palace in 2 weeks but staying at Dreams for their honeymoon. I can see what their experience was at Dreams after they return.
Hope this helped answer some of your questions.