Is being "tangential" a valid objection to a discovery/subpoena request??

Call and tell them you would like to meet and confer. If you are not happy you will need to file a motion to compel. But maybe there is another way to get the information you are requesting.

That is not an objection that I have heard used before, but anything is possible. A review of the response would probably be necessary to determine if the objection is appropriate. If they responded anyway, then it may not matter. Take everything to a local attorney for review.

no that is not a legal conclusion. If you want to discuss the legal authorities for discovery etc. you need to retain an attorney. you will not obtain meaningful advice on avvo regarding discovery because it is too fact specific.