I don't know if anyone will be able to answer your question with a high degree of certainty. You know your planting site better than anyone else and appear to have a handle on the variables. My idea of optimum performance of figs is to harvest nice amounts of really tasty fruits with only marginal care, and without maintaining a tree that wants to take over the world. When it comes to inquiries where limited advise is being offered, sometimes you just need to use your best judgement and then roll the dice. There is something to be said for having faith in a healthy plant's ability to outgrow marginal pest/disease pressure. Actually, why not demand it? Who has time to baby their garden or trees? Your plan to solarize sounds reasonable -- so I'd move forward with it and keep us posted on your results.

DE will scatter. Sand is the better choice. Mix a little Portland cement too. Scott

If you live in New York as suggested in your name, you might want to search on cliffandjoann. They built a stunningly beautiful pond and have explained the details. They say that some of their plants are in pots. I use impatiens and am under the impression that they will grow without much actual soil. It is certainly true for water cress (from the grocery store) anchored directly in flowing water. For something larger, sweet flag will certainly work. You can also use vines or other plants growing beside or near the area. You can also add logs or other natural materials for interest and coverage.

Aka Ivy Leaved Toadflax which is the more usual common name over here.