Your friend has already posted this morning naming the gym... Where is your friend from? Has your friend already lived in Thailand for a long time? Did you friend have 500k seasoned in the bank for a few months?
The interview takes place at the embassy you've applied at, if you've applied in London then you should go there and bring a copy (print it out) of all the documents you've attached to your application and your passport
Anonymous participant if you have had a previous medical history in Thailand, that could surely help. If not, well, then it's a gamble cuz if could be taken either in a good or a bad way. Perhaps you could write a cover letter explaining why you like Thailand and u want a long term visa. You could write a couple of lines saying that you are diabetic and that Thailand is also a perfect place for taking care of your health issues
If you apply in Canada then u MUST stay in Canada till your application gets approved. It doesn't matter then what countries you'll be visiting after u got the visa, technically u got 5 yrs to get yo Thailand. U can also apply in Europe but it depends on the embassy, some will require to be a resident in the country u'll be applying at