I have several comments and questions about your exercise regime.
1-First, it seems like all of the exercises you list are open chain. Do you do any closed chain exercises? This is important as most of the hip abductors function is as a lateral stabilizer of the pelvis during gait and balance while in a closed chain position. I can suggest several closed chain hip abduction exercises if youd like.
2- In my practice I have found that 2 things are essential in increasing strength in MS
first, increasing the emphasis on eccentric contractions. Eccentric contractions are less fatiguing, but can build strength just as well if not better than concentric contractions. Therefore for all of your exercises, I recommend making the concentric phase at a normal speed and the eccentric phase at a very slow speed
Second, I strongly suggest you perform your exercise in an intermittent manner. What this means is that rather than performing a specific number of sets and reps, you exercise until you reach a moderate amount of fatigue, then recover for 10-30 seconds, then repeat. The reason for this is that MS neurogenic fatigue makes doing a high volume of quality repetitions difficult if done continuously. However, if rests are taken, a much greater number of repetitions can be achieved, resulting in a higher volume of work performed, and therefore greater functional improvment. In research i am currently conducting, we are having patients perform SLR's continuously (eg without breaks) and intermittently (taking breaks every 10 seconds after 2 repetitions. The results have been clear that in the intermittent condition MS patients have usually more than doubled the number of SLR's that they can perform.
What are the functional repercussions of your hip weakness? Is your gait and balance affected? Is there a trendelenberg?
Do you have spasticity or contractures? These can worsen the affects of hip abductor weakness.
What medication do you take, specifically do you take baclofen or ampyra?
Sorry to anser you with more questions, but the better picture i have of your situation the more i may be able to assist you