Merging Investment Accounts ?

You shouldn't.  You should keep the ML account in Quicken for historical reference.. the final transactions from ML should consist of shares transferring out, resulting an an account with no assets but with all historical transactions that correspond to the ML account and annual 1099 information.  That 1099 information should not be associated with another brokerage account.

Your M Edge account would be a new account that should begin with shares in transactions moving all shares into the account (and cash balance) - and the holdings should match exactly what your prior ML holdings were.  If the brokerage transfer was performed properly by their back-office, the Quicken download should also downoad all of the proper cost basis information.  All future income/splits/etc will happen in this account ... which will be issued a separate 1099-Composite at the end of the year.

Again, for 1099-Composite reconciliation purposes and more, these should be two accounts, matching real life, not somehow merged into one.  Your reports will combine data from both accounts for your reports.