After cleansing, toning my face pm, do I apply hydroquinone 2% before or after applying rosehip oil on face? For fading Melasma.

Melasma needs long term commitment with skin care and avoidance of skin factors such as sun exposure (Spectrase Sun Block) which has wide UV spectrum and SPF 30+ protection. Accutane sensitizes your skin to injury from sun rays. 
Daily good habits with topicals may well help you chieve your goals with a good skin system ike the Brightening Kit from KareSkin (see link). You may want to start with Melarase and Pigmentation Correction Complex. Scler-X from another vendor is for PIH relief (see link). Apply your rosehip oil with the HQ cream even though RH oil has not been shown to be efficacious for melasma relief ... 
Clear + Brilliant laser can be added once you start your pretreatment with topicals. An Erbium type laser is good for large pore relief. 
Best

I treat melasma every day in my office. The order of the products is not as important as which products you are using.  I prefer Cosmelan for melasma and like the combination of Retin-A and a 5 or 8 percent hydroquinone.  Please consult an expert.  Best, Dr. Green

Melasma can not be completely cured, however effective treatment are possible. This is because your skin is extremely sensitive to UV and even the smallest amount can stimulate your pigment cells to produce colour. The mainstay of treatment is strict UV protection- hats, sunglasses and SPF every 4 hours.

I prefer to use a combination of creams and laser for melasma. Hydroquinone can be used (5-8%), along with vitamin A creams. I usually start my patients on laser (low dose Q switch or Picosure) a few weeks after they commence on creams. Another laser I use is the Clear and Brilliant in the Permea setting combined with creams.

In some cases I combine glycolic AHA peels, and in other cases I use a tablet called Tranexamic acid to help. Melasma treatments should be tailored to each patient.

All the best, Dr Davin Lim. Laser, surgical and aesthetic dermatologist. Brisbane, Australia