Can steriods injections eliminate scar tissue formed in lower eyelid and prevent need for blepharoplasty?

It is not uncommon to have some scarring after this sort of trauma.  The scar can pull down on the eyelid causing an ectropion.  Steroid injections, often combined with 5-FU can soften the scar and help to reverse the ectropion.   It is always best to start with low dose steroid as it can cause some atrophy of the normal tissues as well as softening of the scar tissue.  Doing more than one injection is ok if scar bands can still be felt at 2-3 weeks after the first injection.  Wait at least 4-6 months before doing any surgical revision.     

Generally the dry eye is from the compromised eyelid position and dysfunction of the eyelid.  Usually there is also some degree of motor injury that contributes to further dry eye and disfunction.  The steroid injection will soften scaring but it also causes fat atrophy.  Certainly it is reasonable to have the first of these injections but the need for the others should be determined on the basis of how your eyelid responds to the first of these injections.  Ultimately it is not blepharoplasty that is performed to fix this type of issue.  Lateral canthoplasty is also not sufficient to correct these lower eyelid malpositions.  Generally the reduction of the malar fracture is not complete.  This reduces the support of the lower eyelid and cheek soft tissue.  Correction of these issues can require an orbital rim implant, hard palate graft to the lower eyelid, and lateral canthal reconstruction.  Occasional these injuries are even more complex.  If the eyelid does not respond to the steroid injections and it is not likely to, do not accept surgical reconstructions that are unlikely to be definitive such as isolated lateral canthal procedures.  They tend to use up eyelid resources that are needed for definitive repair.  You should allow at least 6 months before having a definitive repair provided you are not having corneal break down.  You also need more aggressive dry eye management and this should be overseen by an ophthalmologist, cornea specialist, or oculoplastic surgeon. 

That is hard to say.  You can certainly try steroid injection and may help.  It all depends on the degree of eyelid retraction/ectropion and scar tissue present. You can wait about 3 months after the trauma before considering surgery.  See an oculoplastic specialist.