First of all, I am not an expert on credit insurance - nor indeed have much experience of it. However, I do know a bit about supplier management....
The good news is that you have alternative sources of supply for steel, should the worst come to the worst. Therefore, you can focus on this particular issue of payments without worrying about the supply line for your business. Getting credit from new suppliers might be difficult in this environment, though ... but that is a separate issue.
It seems from my reading of it is that the terms of payment were rescheduled and agreed with the supplier at 3k per month. So far, so good.
Then, probably due to many of their clients suffering similar cashflow issues, your supplier has reviewed its credit management process and sought the services of a credit management/ insurance agency. The credit insurer has correctly advised your supplier to put more pressure (including the typical hollow threat of legal action) on expediting payments. Presumably they have provided insurance based on certain assumptions around target credit terms - yours are probably outside their target and they are now trying to recover the situation. Additionally, they have sought extra assurances by seeking post-dated cheques.
So, if the reading above is correct, there is nothing untoward about your supplier changing their terms or demands (including post-dated cheques) on you: it is merely prudent cash management on their behalf. I wouldn't think there is any ulterior motive; nor indeed read much into rumours about settlements that others have made. In the long run, debts really should be paid at some point and not settled for below par value. If you do settle or seek to settle for below the amount due, it will potentially affect your (a) credit rating and (b) your reputation.
Also, in the long run, you don't have to deal with this supplier again - so you can look forward to that! However, don't tell them at this point that you will switch away - that will only get their backs up. Keep them 'warm' about future business and keep the relationship as good as you possibly can - even though they might be the last people you ever want to deal with again!!
At this stage, my advice is to seek specific and immediate assistance from your accountant in terms of prioritising your debts. You probably need to stand back and look at the overall business and your recovery plans and analyse whether your cashflows are being prioritised correctly. (For example, you need to make sure you aren't prioritising cashflow to suppliers who are just shouting the loudest, over, for example, commitments to employees or utilities companies.)
Following such discussions and analysis with your accountant, propose a final payments schedule with them - and stick to it. If this is a bona fide payments plan, based on correct cashflow priorities determined by an accountant, there's really no arguing with that - even if it does go to court!! However, I personally do not believe post-dated cheques are a great idea, as you cannot predict the future and you may regret writing them.
This is just an opinion. Hopefully it'll help. And hopefully some others may have one or two ideas.