I read your post shifrbv and really felt for you. It is a very stressful situation and it seems to be spiraling. I am trying to think what I would do if I were in your position. Mahagogo5 is right in that your stress and the strain on the marriage are very important to address for everyone's sake, and I am sure you know that, but what do you do.
It is very common, in my experience, that one parent can be more involved and therefore informed about a situation involving the children, be it advocating for Giftedness or disability or supporting sport or music. It just happens for so many reasons. It can be that the other parent, without the same appreciation for all the nuances in a situation looks for a quick fix not really understanding what that means. It doesn't necessarily mean they don't care as much but they just don't get everything.
Can you accept for the moment that you are the one with the greater knowledge of your DD's educational situation at this point? If so then take a deep breath and think about what you really want or need to achieve in the next couple of months.
The MAP test sounds like a key priority. If we focus on that, it already sounds like you have the beginnings of a plan. The extra work with Khan Academy and IXL will work and can't hurt. Hopefully your DD will be increasingly independent over time with working on the computer programs and maybe needs less help? If she does need lots of input from you then just set a limit on how much time you can reasonably afford to work on "after school" type activities. Remember any little bit helps so if you can do 15 mins without feeling like it's onerous. No it's not "fair" that it falls to you but it's not about fair really, it's about a need that you are able to fulfill right now and maybe DH isn't. Try not to let resentment, but I get why you feel that way, stop you from keeping your eyes on the goal which is helping DD.
To avoid further strain at home could you accept that the school is what it is for a few months? Is that going to derail anything major? If it isn't then let that lie while focussing on the MAP. It doesn't stop you quietly exploring alternative options. DH may not be ready to discuss them or consider them right now but maybe in a while depending upon what happens with the current school. Perhaps if the senior teacher doesn't prove to be the "answer" he was hoping for.
When we are in the midst of stressful and emotionally charged situation everything can be so overwhelming. I am thinking of you and hoping you can find a way to manage. If anything I have said is not helpful then please ignore.
