I had a similar but different experience. I was flying last minute for a conference and the only seats open were in economy so I purchased 2 seats so I could do work without upsetting my seatmates.

I got my 2 seats and the 3rd seat vacated to be nearer to her family without anyone replacing her. I asked for an extender and made myself a little bed in the 3 seats up against the wall. Belt held me in. I covered myself with a blanket to avoid having an FA question me about it being against regulations.

Great J class substitute.

To the OP, relax and you should be fine. If you do need the extender ask for it. Or ask for it while boarding as when you walk on there are usually FAs at the entry door. It may take them a few moments but it will avoid you having to ask in front of your seatmates and others with nothing better to do than overhear.