i agree with you...most of the time we don't choose the moment we fall ill. In these times, i would play it safe and have a valid insurance for the duration of the stay.
I am going through the process at the moment to bring my gf ( common law) to Canada on a tourist visa and strap on your boots because it's not an easy task. I hired an immigration lawyer in Canada and after nearly 2 months and nearly 2000$..i am in the waiting stage for an approval..or refusal. As Steve mentions, i think most immigration have their opinion of thai ladies and the success rate is very low...and remember, no agent or lawyer can guarantee you a visa.