The clerk might (likely won't) read anything you send in until the hearing. Go to the hearing. Explain your side. The clerk may be inclined to dismiss the matter if you can show you obtained your license shortly after issuance of the citation.
In answering, I will presume that the deposit has not been returned.
GL ch. 186 sec. 15b governs security deposits. You may have a claim for treble damages, attorneys' fees and costs. Contact an attorney immediately to discuss your claim. It may be straightened out in relatively short order with a demand letter.
Your Landlord would first need to obtain a judgment against you before any attempt at attachment is made. Based on your question you may have a potent claim against your landlord for violation the Security Deposit Statute. Here's an article and resources that will no doubt help you, and without the aid of counsel.
East Boston District Court. I created a class for tenants just like you who are trying to get their security deposit back. You may be able to get 3x the deposit back after July 1st. I recommend checking out the class risk free: http://ilawlearning.com/course/get-massachusetts-security-deposit-back/
What a bad exterminator. Then again, what do you expect. You can withhold rent if you want to amp things up between you and the landlord. I think what's important here is getting rid of the fleas. From my experience I can tell you that doing so is not easy. Instead of hiring a lawyer hire an exterminator and plan a mini vacation away from your house.