Solution for Prolonged Abdominal Swelling After Tummy Surgery?

Thanks for the question -
Based on your picture (and please note that a complete history and physical and report of your two previous surgeries would be required for a final plan) it appears that a re-do abdominoplasty would be a good option for you.
Here's why:
It appears that you have some additional skin, you have a scar beneath the umbilicus that would be removed with a repeat abdominoplasty and scar tissue from your seroma/hematoma could be addressed. In addition you may benefit from additional muscle plication. (You could also possible include the drain scars and bring the drains through the abdominoplasty scar next time).
Abdominal contour is dependent on:
Skin envelope
Abdominal fat (subcutaneous and intrabdominal)
Muscles tightness
Intrinsic Swelling
Fluid collections
Skin Envelope -
People complaining of "abdominal bulge" after tummy tuck typically do not have issues with too much skin - that points to issues underneath the skin.
Abdominal fat -
This can be an issue in people who may be a little bit over their ideal body weight at the time of abdominoplasty. Ways to fix this include the tried and true diet and exercise or liposuction. Be sure to allow enough time between tummy tuck and liposuction to minimize complications.
Muscle tightness -
Muscles plication (or tightening) is usually a part of abdominoplasty. Either failure of the sutures or some laxity can cause a bulge. If the change in your contour was sudden or if you felt a pop - think about the plication having ruptured.
Swelling -
This can be the most common cause of contour issues post-abdominoplasty. Patients typically have swelling for two reasons. First surgery causes trauma and the body's response to trauma is to bring additional fluid into the area that has been traumatized. In addition, abdominoplasty involves dividing lymphatic channels (very very small conduits that normally pull fluid out of tissue) - until these channels grow back the area will not be able to remove excess fluid.
It is usually the most dependent (lowest) part of the surgery that stays swollen the longest.
Fluid collections -
Seromas (serous fluid collections) or hematomas (collections of old blood) can cause distortions of contour. Normally this will improve on its own but may require drainage.
I hope this helps.

The presence of a significant abdominal bulge several months
following abdominoplasty surgery should be aggressively investigated.Especially if the bulge wasn’t present in the
immediate post-operative period.This
type of bulge could occur for a variety of reasons including swelling, seroma formation, encapsulated hematoma,
residual muscle weakness, recurrent separation of the muscles. Intra-abdominal
fat, excess subcutaneous fat, hernia formation and possibly fat necrosis.Under
these circumstances, it’s appropriate to obtain ultrasound and possibly MRI
studies.Depending upon the results,
revisional surgery may be indicated.For
these reason, it’s important to consult your plastic surgeon.Your surgeon should be able to formulate an
appropriate treatment plan.

It is truly difficult to assess your condition from the information given and it appears that your surgeon never provided you with an explanation for the swelling. From his/her recommendation, it sounds as if it is due to fat deposits as indicated from their advice for staged liposuction at a later date which may be a reasonable approach.