In PA most agents use Pennsylvania Association of Realtor forms that are pre-printed and approved for use. If this is DE i am sure the Delaware Association of Realtors has similar rules. If you are buying through a builder they may insist on their own forms.

Most addendums we use in PA are already written as forms, so the answer is, it depends. Some brokers have their own addendums, as well. There are rare occurrences not covered (very rare) that need special wording and an agent shouldn't be playing lawyer with those. The agent shouldn't be making any changes without the buyers knowledge or consent (in writing) after a buyer signs. If you are using an attorney they should have full knowledge of changes, as well, and if they write the addendum it will most likely be on their letterhead. A buyer's agent should be in communication with their buyer and give copies of what is sent to a seller/listing agent, both before and after the seller has signed, IMO.

If you have an attorney let the attorney handle addendum's and modifications. Your agent should not be changing any addendum items without your knowledge. Make sure you know what your are signing and exactly what is on those addendum's.