70, fit, BMI 21, tummy tuck & past lipo, I don't know if I should have muscle tightening? (Photos)

Yes I recommend you have muscle tightening with your tummy tuck as it will give you a much better result.
There are two major steps in tummy tuck (and many important minor ones) - removal of excess skin and tightening the muscles and you will benefit from both of these steps, in fact I wouldn't recommend doing the tummy tuck without the muscle tightening in your case, as you demonstrate very nicely in your photos the difference it will make for you.
I would have no concern doing a tummy tuck for you 20 years after liposuction, The scars in the areas treated with the liposuction will be soft now and won't present a problem. You have a smooth tummy now and can expect a smooth tummy after, unless you have a complication (for example, infection). Yes experienced plastic surgeons deal with this all of the time.
The only challenge compared to an average tummy tuck is that you are 70, and as we age, we don't bounce back from things as quickly as when we are younger. However, the fact that you exercise and have a BMI of 21 also goes a very long way to predict that you will have a good recovery. This comment on your age is a scientific fact, and not a judgement. My advice is make sure you get enough rest after surgery yet be sure to be up and about every day, and I expect you will do great.
I think it will be a good idea for you to do some more consults.
All the best to you,
Dr. Nancy de Kleer

Thank you for your question and photos. I have performed many tummy tucks on women who previously had liposuction to their abdomens. Many women have poor results following liposuction, and a tummy tuck will release the scar tissue. This will improve your contour, and significantly smooth many irregularities caused by liposuction. This is a fairly common problem that can be corrected with a tummy tucki.

Dear Firstkitty,
Yes the lipo does pose some challenges during elevation of the skin flap. That just means the surgery goes more slowly and incrementally to stay in the right plane with cautery dissection. Other than that, you obviously have some laxity of the abdominal wall, particularly the lower. That will be corrected by bringing the rectus muscle fascia to the midline, from top to bottom to make it even from top to bottom.
Your skin may not be as stretchy in the vertical direction due to the previous liposuction, so be prepared for the umbilical donor site (hole) to be repaired as a small vertical incision in the lower abdomen. That will be determined at the time of surgery.
Your age is fine, no impediment to surgery.