F1onajb - you can, but just beware that the carb counts vary widely for tinned tomatoes. The lowest I have found are Sainsbury's Basics, and they come in a square tetrapak. Have you had a look at the lists of food on the spreadsheet? I can't remember exactly what's on there, but I'll have a look and see if I need to write more. But if you go to the OP on the week 1 chat thread, there's a list of the main stuff to avoid.
weaseleyes - it's difficult to be that prescriptive because it depends entirely how much of the food you're going to eat! If it's something like, say, English mustard, you'd only be eating a very small quantity, whereas if it's something like coleslaw you could easily polish off 100g. As a general rule of thumb, I'd say anything under 4g carbs per 100g, but obviously take into account the quantity
pingufan I always buy tuna in oil, preferably olive, and then add mayo as well!
zoodle - if you've reverted to eating a high carb diet, then yes it's likely that you will go through carb flu again. If you're feeling really horrible, then eat more fat, drink more water and make sure you're getting your electrolytes - you need sodium (so cook with salt, add salt to your food and try a hot drink made with Oxo/Marmite or Bovril), magnesium and potassium, so eat salmon, spinach, avocado and full fat natural yoghurt
pregnantpause I have never used MFP so I can't comment specifically - but I do know that other people have had an issue with the carb counts on there as well, so I have continued to avoid it. I have a couple of carb counter books, but they can contradict themselves as well, which is a bit difficult! Generally I Google things - so 'how many carbs in a XYZ' - but make sure that I only look at results that come from a UK site