Blimey! Give the poor woman a break! You tell her you can't comment without further details and then lambaste her when she provides them. She can't win!
OK OP I am going to try and help you, note the word HELP and not criticise or be down right rude!
Firstly. Your partner, if as you say has had a drop in income, can only pay what he can afford to pay. End of. As someone has said on here, who I applaud for their great common sense, your partner and their ex were still together they would have to ride out this drop in income. The same applies even if they are no longer together.
It is unfortunate and I would hope that once your partners income increases, if it should increase, he will then revisit the maintenance and address the figure upwards.
Secondly, ignore the threats of solicitors and courts. They will highly likely be empty ones. Should this turn out not to be true, then the court will look at incomes and say much as I have said in the first point i.e. his income can only support so much maintenance, end of.
Thirdly, it is common for variations in maintenance to cause stress for ALL. Things will calm down. You just have to ride through it, all of you.
Lastly, you have been in this relationship for what a year? It's early days. I understand your frustration and disappointment with the ex's response at this stage. I really do. However if you love your partner and your stepchildren, you will ride this out, suck it up (meant in the nicest possible way) and understand that it's not about your partner, you or your ex but the children who you will hopefully grow to love and care about (if you haven't already). You will learn that you do not want them to suffer and that providing two happy homes in which happy adults exist in whatever way you can (and sometimes that means throwing a lot of money at it for all the right reasons, and in my case some of my own even; missing out on social events; and romantic nights in favour of investing emotionally and financially with the children etc etc such is family life), will provide the best environment for these children to thrive and feel secure and loved. It may take a wee while to get this and to move on from feeling some resentment towards other peoples actions and behaviour but it is in everyones best interests if you do, including yours ;-)
Love your partner, and his children and support him in doing the best for his children and take the ex out of the equation should you need to (and I should add here that many ex's are lovely) and you have the basis of a long lasting, happy relationship.
Good luck from a stepmum of three years who has been through and seen a lot, learnt a lot and loving it.