I don't know the answers to your questions, but have you tried looking up the types of foods you are supposed to avoid? Generally they are classed together. I know I have to stay away from large quantities of nightshades, they aggrevate my joints- that's potatoes, tomatoes, peppers, eggplants and cayenne. Most of your list is considered gaseous forming but not necessarily causing other issues unless you have identified it as an intolerance. The classification might give you more clarity.

As for soy sauce- take a look at Bragg's. Its gluten-free. In the states it might be billed as coconut aminos?

I would use leek in place of onion, if its allowed? And I would sautee whatever you use to increase the flavour. I might also try fresh sage leaves if they are ok, or fresh parsley. Change the leafy herb based on the dish. Chopped kale is also quite tasty and can round out a flavor palate- but it could fit under cruciferious veggies category...

Wheat was my big trigger. I will say though that it took several months for the wheat to completely clear my system. If its that, then your gut in theory should start to heal allowing you to maybe add things back in. Go back to the basics- foods with only one ingredient on the list. As you get used to it, its easier to appreciate the simplicity of the individual flavours?

I think lemons and limes are ok in low doses on the FODMAPS - adding those juices to your olive oil for a marinade seems like it would be a good option.

Also, at http://www.todaysdietitian.com/newarchives/030612p36.shtml, they suggest onion flavor can be gained with chives or the green part of scallions. For garlic they recommend sauteing whole garlic cloves in oil and then removing them, to get the flavor without the high FODMAPS part. There seems to be other interesting tips at that site, too. Good luck!