Oh boy! Quite a tricky situation @Erica Juh
Not legal advice. I am not an attorney, no-sir-ee-bob!
I have a few questions for you to get the answers for. Find out from your attorney if the Seller is able to close a transaction with another Buyer, if their is an unresolved earnest money dispute. Find out, because this can give you leverage to demand a refund, even if you are not really entitled to one.
If I was your Realtor, I would have seen your letter as a mistake, and would not want to deliver it. You are essentially telling the Seller that for reasons that are not contingencies within your offer, you want out of your contract. To me, this admission may disable you from getting your deposit back because you are naming reasons for renegotiation outside of your contingencies. If your attorney could not find a reason for you to back out of the contract and get your deposit back, then and only then, would I be fine delivering your letter. Obviously, if my client insisted I deliver such a letter, I would, after explaining my objections. If, after hearing my objections they still wanted me to deliver it, fine.
The second question for you to get the answer. Did the Seller provide a Property Disclosure Statement and identify the code issues to you (A disclosure is a 5-ish page document that the Seller can share all information they know about the property to you)? if they didn't disclose the code issues, they could have some culpability. That may have been your way out, depending on your disclosure review period and the date when you were provided the disclosures documents. OR did the Seller provide a disclaimer (a single page document that says......essentially, I know nothing about this property, you figure it out)? In my State, if a disclaimer is provided instead of a disclosure, you may have the right to back out of the transaction up until closing (unless it is a foreclosure, which this doesn't sound like). You would have had to sign either a Seller Disclosure or a Seller Disclaimer, because your signature acknowledges receipt. Your Realtor will know which one was provided, as he would have gotten it to you for signature. if neither has been provided, that may be your ticket out as well. This disclosure or disclaimer should detail your review period of that specific document.
As far as being mad at your Realtor for not putting a due diligence period in your contract. Due diligence periods give you a reason to back out of a contract for ANY reason, so that is actually much more of a broad contingency than just an inspection period. In a competitive situation, I can see why your Realtor did not assume you would want it in your contract if you were even waiving an inspection period, it would actually be counter-intuitive.
I don't believe the Seller would deny signing an extension, after all they want to close as agreed. However, I can see they are not interested signing an extension if you just want to renegotiating the terms of the contract. You made it clear in your letter that you are interested in renegotiating the price. If they are not wiling to reduce the price, why would they feel inclined to extent their obligation to you? It is not odd that your Agent has not gotten an extension signed, I can't imagine why the Seller would agree to an extension at this point.
Lastly, but maybe most importantly: You are clearly uncomfortable with your Realtor. Find another Realtor to discuss listing the home with (today), while you still have time to close. Meet them at the house and ask them to tell you what they think they can sell it for once you complete your repairs and upgrades. You might be pleasantly surprised, and encouraged to move forward.
When you are new to flipping, you should not be writing high risk contracts. Start off feeling the water before jumping in. =). Meaning, start with inspection contingencies. Maybe ask for 5 days instead of 10, but still have them.