First things first. USDF does not have a rulebook under which dressage tests are run. The USDF runs its tests under USEF DR (dressage) rules. So those are the rules that apply to your bridle. And a Micklem bridle is indeed legal under USEF DR rules.
If this HT is being run under USEF rules, you are good to go with the Micklem.
EV115.2a says: The following are compulsory: an English type saddle and a permitted bridle. So the phrase "permitted bridle" means to me any bridle permitted by USEF DR121 When you look in the USEF DR rulebook, you find this in page 31:
Note the last sentence, which answers your question re bit clips.
| 6. Micklem Bridle. Permitted for use at any level where snaffles are permitted. Not permitted for use as a double bridle. Bit clips may not be used with this bridle. |