If the exchange rate or fees are estimates, do the disclosures to the sender have to explicitly state that the exchange rate is an estimate, or the fees are an estimate?

Yes - in looking at §1005.31(d) and the commentary, it requires that estimates be accompanied by the term "Estimated" or something similar:

(d) Estimates. Estimated disclosures may be provided to the extent permitted by § 1005.32. Estimated disclosures must be described using the term "Estimated" or a substantially similar term in close proximity to the estimated term or terms.

Commentary:
1. Terms. A remittance transfer provider may provide estimates of the amounts required by § 1005.31(b), to the extent permitted by § 1005.32. An estimate must be described using the term "Estimated" or a substantially similar term in close proximity to the term or terms described. For example, a remittance transfer provider could describe an estimated disclosure as "Estimated Transfer Amount," "Other Estimated Fees and Taxes," or "Total to Recipient (Est.)."

The model forms also give some examples.

Thank you!