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Invisalign and braces in general work best when a constant force is applied to the teeth causing tooth movement. If you are not compliant in wearing your Invisalign trays constantly then I would not recommend them. Your teeth will be in a state of shifting during the evening and shifting back during the day when you are not wearing them. Your treatment will either progress very slowly or not at all.
Invisalign works by having constant force on your teeth for 20 hours a day. You essentially act like they're glued to your teeth like braces would be. If you sleep in them only, the teeth will move at night but will move back to where they were during the day resulting in net-zero.
Did the pain start after inserting the new tray? IF so, your teeth are not quite ready for the next tray, you may need to go back to the previous tray to allow gentle tooth movement to take place for more days. If the pain is due to pressure from the new tray, the pain should subside the moment...
Yes Invisalign should work well for your case and you would be able to achieve alignment of that one flared tooth as well as rounding out your smile in general and aligning the rest of your teeth as well. 
Probably still fine but you should make plans to get a replacement in the next few months. It is easy to duplicate the retainer with just a phone call to the dentist who treated you.