Can a canthopexy be performed a few weeks after a blepharoplasty to correct a mild lower eyelid ectropion?

Dear Scared
First being only a few weeks after surgery, it is very difficult to know how you will actually heal. Some things get better and some things get worse in time.
Second, lets get our terminology correct. An ectropion is an outward roll of the lower eyelid such that there is a gap between the eye and the eyelid margin. This situation is relatively rare following lower eyelid surgery and would generally be caused by the surgeon removing too much lower eyelid skin in the setting of a very lax lower eyelid. The laxity of the lower eyelid should have been part of the surgeon's pre-operative assessment of the lower eyelid.
While it is possible that you have an actual ectropion, it is more likely that what you have is a post eyelid surgery lid margin contour abnormality with vertical inadequacy of the lower eyelid and possible lid margin roll. Outward lid margin roll results in visibility of the lower eyelid margin when you are looking directly at yourself in the mirror. This is not frank ectropion until the lid margin is pulled away from the eye. You may be confused by the terminology because many general and facial plastic surgeons refer to any eyelid contour change after blepharoplasty as "ectropion."
Canthopexy is a procedure that simply reinforces the strength of the lower eyelid ligament. Canthoplasty is a more robust procedure that shortens the ligament, repositions the canthal angle, and has some effect on the shape of the lower eyelid contour. However, neither of these procedures by themselves will correct a post blepharoplasty eyelid contour abnormality. For this, it is sometimes helpful to massage the eyelid upward while it is healing or place the corner of the eyelid on stretch with a supporting suture. I tend to avoid steroids because of the fat atrophy that can develop but 5-FU can sometimes be used to successfully modify the scar that is developing in the eyelid and likely to make your situation worse as healing evolves.
Ultimately if these measures are not helpful and after 6 to 12 months you are still not satisfied with the lower eyelid contour, post-blepharoplasty reconstruction with hard palate graft can be performed to restore the desired lower eyelid contour.

You should definitely wait at least six months prior to considering a revision surgery. There are a range of conservative measures that should be considered by you and your surgeon during that period of time. These include taping, massage, and possibly a temporary suture to help support the lower eyelid.

The robustness of your lower eyelid support should have been a very important part of your pre-surgical evaluation. This is not to say that your surgeon did not consider this issue. All surgeons that have done a significant number of blepharoplasty surgeries have encountered patients in which the impact on lower eyelid position has been greater than they might have expected. The key thing is to make sure your surgeon is taking a detailed history and physical exam prior to surgeon.

Ectropion is a result of many things during lower lid blepharoplasty, removal of too much skin, scarring of the orbital septum or damage to the muscle. However, during a normal blepharoplasty we do things that we know will create ectropion so other things must be done to counter this.
A canthopexy is part of the procedure to fix ectropion. The second thing that must be done is elevation of the midface to reduce the downward forces on the lower lid.
However, you're too early for any of this. You should be doing massage of the lid and taping to start. After this your surgeon may consider steroids to help. If it's not better in 6 months then look into surgery.