A written statement of potential legal actions to follow should an agreement not be reached is acceptable within a letter from an attorney.
But the ‘threat’ of legal action is not the same as the actualization that the Court will agree with and enforce the threatened stance.
Whether or not those ‘threats’ are accurate, or legally correct, or will play out as described is another matter all together; your own legal counsel can advise you as to your best course of action based on the content of the letter.
If you are not 100% sure of all the corresponding laws regarding the matter in question in such a situation, a pro se handling of the response would not be wise.
For advice specific to any personal legal questions, Standard Legal always recommends you consult a qualified, licensed attorney; you can find a local attorney for FREE here.