You have not shared your educational qualification / work experience details...
The Interim maintenance amount is awarded a year back, if you are unhappy with the amount then why don't you immediately start looking out for a job and support for the upbringing of your children. They are 12yrs old now, so cant understand why you have to stay unemployed, if you look around there are hundreds of women who keep working despite hectic job schedules...... also there are many work from home job options available ....from your language you seem to be a well educated women.
Im not taking sides as im unaware of the merits of the case, but please understand and accept that your husband has produced all necessary legal documents which are accepted and hence IM is awarded. Now if you think your husband doesnt want to pay you more what do you think is the best /immediate/ alternative ??? to keep suspecting/complaining or start contributing for your kids ?
Choice is yours mam.
Also one hard fact is that most lawyers (though not all) keep telling women clients that they will harass husband so that he pays up higher maintenance but there are smart and cunning husbands who know how to reverse the tactics and make the women run for money.
So for betterment of children i suggest take up suitable employment, your husband will support for kids and he has to by law no matter whether you start earning