I Have Been Wearing my Expanders for 2 Weeks and Activated Them 14 Times(3.5mm)

If you are diligent with your wearing of aligners and then retainers when you are done, arch expansion should be maintained.  Teeth will move only if there is a force acting upon them (in this case, the cheeks).  If a retainer is rigid enough to resist these forces, you should be fine.

But you will need to wear a retainer for the rest of your life.

You are wise to ask these questions. At your age that rapid of expansion likely it only tooth tipping, which maybe all you need. If that is the case Invisalign will be able to maintain that with excellent compliance on your part.

You clearly chose your doctor because you trust his expertise. As long as he is a certified Invisalign provider and has many cases under his belt I am sure you are in good hands.

Chances are, your doctors has done this before. Expansion at your age will result in dental tipping, which usually will relapse quickly. However, if your doctor is able to move the teeth out a certain distance, your corrected bite may keep the result stable enough until your recieve your aligners. It's hard to say without looking at your case, but ask your doctor, I'm sure he or she will have a great answer for you.

Palatal expansion can enhance your Invisalign result and can be accomplished at any age with a removable appliance that pushes against the palatal bone,  is activated slowly (2 X a week), and has good stability.  However the most common type of expansion called Rapid Palatal Expansion (RPE) calls for a more rapid activation (daily),  has been shown to be more unstable, so if you had this type of expansion, plan on some relapse.  However, your orthodontist is giving you good information in that the aligners can be made to hold the expansion you have gained until more stability is achieved.  Wearing retainers at night indefinitely after finishing your Invisalign treatment will assure you the best result.

At age 21 there is a good chance that the suture down the middle of your palate is not separated. It could be separated but there is a large chance that the expansion all came from tipping the teeth towards your cheek. If the suture is separated it is wise to leave the expander in for three months to allow bone to fill in the suture. If there is no separation of the suture then waiting as long would not be as beneficial. A good indicator that the suture was separated is if you now have a space between your two front teeth you didn't have before.

Either way, the day you remove the expander you should have a clear retainer made to hold until you receive the first Invisalign tray. In your case the retainer tray should not be horseshoe shaped. The retainer should cover the roof of your mouth to give more strength to fight the relapse potential.

If you are a mouth breather once treatment is over you should plan on wearing your retainer more diligently than most the rest of your life. It is probable you will have more of a tendency for relapse than others. Hopefully night only wearing will suffice.