Corn is not as bad as wheat in terms of gut irritant and allergenic/inflammatory compounds. I would be fine eating 1.5 cups of corn. I would try to avoid corn meal though like tortillas.
If potatoes (and sweet potatoes) are boiled rather than baked, their GI stays "medium". The high heat cooking like baking breaks up the resistant starch that is healthy and slows down absorption. Boiled plantain is similar. Anytime you are browning or cooking with dry heat, or frying the GI will go up (although the oil will slow down absorption also.)
Personally, I stick with white rice combined with a fat, and a little vinegar to slow down absorption. White rice as part of a mixed meal is not likely to spike blood sugar. I tested myself with 1 cup of white rice by itself, and also as part of a mixed dish with butter, egg yolk, mushrooms and onions and a splash of apple cider vinegar. The plain white rice got my peak blood sugar up around 130, while the mixed dish peaked out at only 110. I checked every 30 minutes for 3 hours. Also the plain rice ended up putting my blood sugar in the 60s after 3 hours while the mixed meal stayed in the 80s-no crash.
Rice, Potatoes, Plantain, Sweet potato, not overheated, and eaten in mixed meals with fat, protein, and veggies.
A couple of points. You have two goals. One is to prevent your pancreas from overworking 24/7. This is because it reduces your insulin sensitivity, and eventually causes a cycle that burns out the pancreas. The other is to keep blood sugar peaks under 140 because blood sugar over 140 destroys pancreatic cells.
The way to prevent the pancreas from overworking is to not eat too many total calories, lose bodyfat, exercise daily, sleep well, and avoid inflammatory foods. It is not really the result of reducing total carb content in a diet or GI. In fact, low GI meals may require more total insulin than high GI foods.
The way to avoid blood sugar spikes is to eat more around activity, eat mixed meals with fat, protein, veggies and some acid with carbs, and to also do everything in list 1 because if the pancreas is not overworked it will do a better job of protecting itself from high blood sugar.