oh goodness admylin, I do feel for you. Any marriage problems must just magnify the integration problems. Have no idea what is the best thing to do.
Motivation talk with the teacher: well, she is young. I breezed in, wearing my most intimidatingly smart clothes. I knew, that her line would be that ds is the worst in her class (could be true), and I wanted to avoid the position of either trying to defend the undefendable, or simply apologising for him. So, I let her tell me everything about how terrible he is. then I told her (and kept repeating) that ds is terrible because he isnt motivated. and he isnt motivated because his integration into the german school system was so appalling, (for which they must take some blame). and that I pay for 5 NH sessions a week (ie I am a concerned parent). and that her telling him off so much every day was merely making things worse. eventually I needled her enough that she admitted she wants to give up on him (which was obvious). so I said sweetly "but you are the teacher, you are a trained professional..." and she laughed. If she hadnt laughed at that point, it would all have gone titsup, I think.
then we started talking about how to improve ds's performance in the school.
Ds's motivation tutor actually came to visit us today. She spent a couple of hours with ds, and after she had gone, I saw the old ds again. He whizzed through the remainder of his homework and has now gone to bed (allegedly).
Re persuading your h about Nachhilfe; he is just in the denial that every foreign parent goes through when they arrive in Germany. We are so used to the free state school system, that it comes as a shock that you have to either half home educate, or pay a tutor. Especially as nobody tells you this.
If I had started with tutors right from the start, we wouldnt be in the mess we are now in, I am sure. But the sums of money involved seemed horrific to me at the time. If he is worried about the money, can you get someone who is paid cash, point out to him its cheaper, etc?
DD has got her mid term report too: the teacher says she is going to give her 2,33 average. this will give her a choice of either repeating the 6th class in the RS (very bad, she wouldnt lift a finger all year, why should she), the Wirtschaftschule (she'd have to choose her future job, and then basically have lessons only in that, eg bookkeeping instead of maths.), or staying in the HS and trying to do the Uebertritt again next year.
I will try to persuade the RS to accept her in the 7th class, even on a probezeit (heavily backed up with NH, of course), but am not optimistic. I dont want to rub it in and add stress, but if she does well in the reading comp, that might make all the difference....!