I don't believe it is too late to plant fig trees now when purchased as 'potted plants'. However, it may be a bit late to get choice potted plants from your local nursery but try anyway. Also, your local nursery may be able to recommend what are good varities for your area.

1/4 to 1/2 inch in diameter, maybe a little larger -preferably woody. Take a 1 gallon pot and make a cut down one side from the top to the bottom. I usually extend the cut into one of the drainage holes which is usually larger than the diameter of the branch I want to airlayer. Wrap the pot around the branch so that the branch is passing thru that drainage hole. Then tape the cut pot wall on the inside then on the outside.(I use duct tape). Reinforce it real well with tape. Fill the entire pot with moist potting soil and pack it down firmly down to the bottom of the pot. Keep packing in the soil until you get to within an inch of the top of the pot. Water it well and let the water soak in the packed soil. Then ball up some plastic bags from the super market and pack them up on top of the soil in the pot to fill that 1 inch gap. This will keep the soil from drying out too fast. Secure the plastic with some foil or tape or rubber bands. Each week, take the plastic bags out to check the moisture in the soil. If still wet, replace the plastic bags. If dry, sprinkle some water on to of the soil and let it soak up, then place the plastic back on top. You should have roots in 3-5 weeks. Now here's the great trick about doing this in a pot versus a plastic bag. Since it's so late in the season, just keep that pot in place until the tree goes dormant. Keep checking the moisture of the soil in the pot. Don't allow the soil to go dry or else the roots will die. After the entire tree goes dormant, separate the potted airlayer and store it where it won't freeze. You've got a potted tree ready to repot into a larger pot in the spring.

If you provide more information (and a picture), you may get more accurate information. Either of the two issues listed above could be your problem, but its hard to tell because some nutrient issue show up in a similar fashion to leaf rust. If you are dealing with a nutrient issue, more information would be ideal in pinpointing which nutrient is giving you problems and, more importantly, what is needed to correct the problem. You mentioned some of the leaves fell first then the remainder fell later. Was there a pattern to the defoliation? The answers asked above will also give insight into your problem.

As a general rule you want to plant perennials (such as trees and shrubs) either in the spring so they have time to establish roots to take them through the summer heat or you want to wait until the summer heat dissipates and then plant in time for the roots to establish enough to take the plant through the rigors of winter freezes. Either way works fine. What you do not want to do is plant in the mid summer when the plant will lose water via transpiration faster than it's roots can take from it's new home. This just results in stress which weakens the plant heading into winter. As far as preparing the soil, don't bother. It sounds like you will be planting a lot and won't likely be able to baby what you plant so modifying the soil is a losing proposition. You want plants that will respond well in the existing conditions with no babying. Yes, you can start shrubs/trees from seed, just understand that the small ones in the nursery are likely 2 or more years old. In other words they take years from seed to amount to anything, but if you have the time seed is more economical. Yes, you can container grow and transplant in the fall without a problem, but you will face higher water/nutrient demands in the container than you would the soil so why wait?