1) I accept message only after a signature is validated successfully. It is unlikely to happened that someone send a correct message (correct in term of signature) had set unknown action value. Because the one who send a message with correct signature is most likely the owner, and the owner must be aware about actions his smart contract supports. In case the owner indeed send unsupported action, the smart contract will throw an error, which is OK in me opinion because it indicates a specific error 32. This way the owner knows what went wrong on his smart contract during execution.
2) I wrote this code basing on existing ton blockchain code, in which error is being thrown (https://github.com/ton-blockchain/ton/blob/master/crypto/smartcont/config-code.fc#L123) after accept message (https://github.com/ton-blockchain/ton/blob/master/crypto/smartcont/config-code.fc#L103).

> - subdomains can't be deleted...

I don't delete subdomains, please clarify where do you see an issue.

> - dns-resolver-manual-add-single.fif only supports addresses as values

Correct, but again what is the issue here? TON_Contest.txt does not specify format of DNSRecord, it just specifies an output format, saying that smart contract should return a cell containing DNSRecord if category has found, or cell containing the an entire dictionary (HashmapE 16 ^DNSRecord). I've met these requirements I believe.
As I specified in the README, I choose to hold addresses within DNSRecord as this is the simplest in my mind to demonstrate how my smart contract works. Obviously, dns-resolver-manual-add-single.fif can be easily adjusted to any DNSRecord format, once this format is clarified.

Thank you for the review.
Let me reply on you remarks.

- There is no way to completely delete expired domains
I focused on functionality which is required by description. There is nothing in TON_Contest.txt about deletion.

- Returns number of bytes instead of number of bits.
Right, this is the way I understand requirement in TON_Contest.txt. Now, when I read it again I see that the text is quite confusing and number of bits is expected.
"If the subdomain is found, the smart contract should return (8m, dns-record-value), where *m* is one plus the number of bytes in the subdomain found."

- Ability to change code of an inited smart contract is very dangerous.
Agree, I should have delete this functionality before submitting the code, but it really helped me a lot in debugging.

I agree that delete operation and expiration time for each record should be implemented in a production ready smart contract, but following strictly by TON_Contest.txt, expiration time is mentioned in the requirement of Automatic DNS, not Manual DNS ("...or to extend the registration of the subdomain. A special get-method may be defined to inspect the expiration time of any registered subdomain.").

Perhaps, I treat requirements too strictly and should have apply a common sense, however I took the way to implement exactly what is requested (I'd even say nothing more than requested) with the best quality I can.