Please give a medical explanation of how a hematoma heals, how long it realistically takes & possible effects on final results?

It seems that you're unhappy with your recovery. To answer your one question, usually hematomas are not common. It is a  known complication that can occur after a neck lift or face lift procedure however. They are more common in people who have uncontrolled high blood pressure as well as male patients.
Small hematomas usually resolve on their own. A large hematoma or an expanding one must be surgically evacuated. Hematomas do cause a delay in the healing process but a lot of times those patients end up with extra scar tissue underneath and actually end up with a firmer appearance where the hematoma was.
With newer techniques in my practice, usually I do not use any drains after any facelift or necklift surgery.
If it does not settle, you can undergo correction with aspiration or with additional surgery.  I would recommend that you see a board-certified plastic surgeon in your area that is a member of both ASPS & ASAPS and performs a lot of facelifts if you want a second opinion and of course, make sure you follow up with your own surgeon and talk about this. 
I hope this helps and wish you all the best in your surgery and event. If it does, please take a moment to up vote this answer on the “respond” button on the top right.  

At 3-4 months after surgery small hematomas and edema has mostly resolved.  If you still have irregularities it may be due to scarring or scar tissue related to the fluid collections.  You may require a small revision to recontour your neck. 

It is not uncommon to have this sort of edema and swelling post operative from plastysma work. It is a broad muscle and swells significantly. I can only recommend seeing your surgeon asap to get their recommendations as some change depending on what was done. I council all my patients that need platysmal work that the recover period is long and frustrating. This is why we have developed a unique liosuxtuon technique for the neck to avoid as much as possible this issue. 
Best of luck     DrC

It is difficult to answer your questions without photos.  However, a hematoma left undrained can cause the issues you describe.  Go back to your PS for a detailed explanation of what is going on.  If you are not satisfied, find another PS.  Your skin and tissues can become woody and fibrotic if left too long.

Small facelift related hematoma's are usually absorbed within a few weeks. When they do heal though, sometimes they heal with an area of organized fibrosis, which can feel lumpy. Small areas of swelling can fluctuate, but by 4 months or so it's almost gone. Lumpyness and contour irregularities can be from several different reasons: irregular fat removal, bunching of the platysma, and uneven dissection.  I would consult your primary surgeon to see what needs to be done. Sometimes a revision is needed, but at times small steroid injections can be helpful for contouring. 

 An untreated hematoma will ultimately become organized inducing fibrosis and contour abnormalities. Overtime this will substantially resolved but  may result in a situation that would require surgical revision. It's best that these are treated early.

A  Hematoma is a known complication that can occur after a facelift procedure, and is more likely to occur on a male facelift, than a female facelift. In our practice we use  Jackson Pratt drains to help prevent it, but they can still occur in the first 24 to 48 hours after the procedure. Small hematomas are addressed by placing pressure on them and they resolve on their own. A large hematoma must be surgically evacuated to remove the  blood clots from underneath the skin. Although there may be a  slight delay in the healing process with lumps and bumps underneath the skin first few months,  most patients resolve quite well

A hematoma by definition is a collection of blood. All hematomas should be addressed when they happen in cosmetic surgery . If not, it will be left up your body to take care of the blood pool. Your body will, over time, process the blood pool, usually leaving scar tissue in the process.  A similar process occurs in blood around breast implants which may result in a scar contracture around the implant.