In my 13.5 years on this forum, I have never heard of anyone being able to extend a fiancé visa. It’s essentially impossible.
HOWEVER, given the current unusual circumstances, I assume they will have some provisions in place.
At the moment, though, they are only allowing visa extensions out to July 31st, so given that your visa doesn’t expire until October 19th, all you can do is wait and see what happens. I assume that if there are still issues by the end of July, they will allow further extensions (though a full 6-month extension is very unlikely).
Bear in mind that you only have to submit your online application for FLR(M) by October 19th, you don’t need to have the visa processed by then, so that still gives you until September to give notice and still be able to marry and apply for FLR(M) before October 19th.
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