To address the last part first, it sound like they got insurance but you had to remind them. Unless you were damaged by their failure (a claim arose while the home was uninsured, for example) this may bug you, but it is not an issue the courts will fix.
The wording of the contract controls the payments: if it clearly said $5000 down then a payment in July and monthly payments due on the 5th of each month thereafter, then I agree it sounds like they are behind one month. It will cost thousands to hire an attorney to prosecute a suit to recover that one month's payment: the court can award reasonable attorney's fees along with the missed payment and any contractual late fees or interest at the conclusion of the case if you win.
I wonder, and the judge will wonder, why you waited a year and a half. Breach of contract is enforceable up to 6 years, but if the defendants can show that you waived the breach, they would win and you would need to pay their attorney's fees.
The general advice above does not constitute an attorney-client relationship: you haven't hired me or given me confidential information by posting on this public forum, and my answer on this public forum does not constitute attorney-client advice.