Will invisalign retainers work the same as a mouth guard for teeth grinding at night?

No. A properly fitted mouth guard for clenching/grinding purposes will be fitted to your bite and problem associated with your teeth grinding habit. Invisalign retainers are much thinner and do not take into consideration the way your teeth come together when they are in so could actually make a person who suffers from clenching/grinding, clench/grind more when wearing at night and grind through them.
No. Don't expect your retainer to help you with grinding at night. Talk to your dentist about a night guard. This will help your grinding and also serve as a retainer and prevent your teeth from shifting.
Yes. You tend to grind less when your teeth are being moved with the aligners. But once the ideal position is achieved you will need a definitive mouth guard to reduce your tendency to grind. I recommend nti device to all my patients with your situation. It can be fitted over your final retainers.
Limited. Although clear aligners or retainers provide a modicum of plastic thickness between teeth, they may not be the ideal replacement for a bite splint. Best to ask your orthodontist's opinion for your situation.
No, too thin. Invisalign aligners are designed to be used 2 weeks a time. We see some that have been worn for up to 4 weeks and they are looking tattered. You will need to have a consult to determine your individual needs. Night guards are typically made of a much thicker and sturdier plastic that can withstand the wear and tear of night grinding.
Night guard. Invisalign retainers will not act as night guards. Night guards are made of thicker material to withstand night grinding which is is very powerful. If you use invisalign retainers as night guard, you will grind through them and destroy them. I hope that helps.