Are you sure the Central Americans would have been happy under Mexican rule.
Central America was part of the Viceroyalty of New Spain but they were made there own Kingdom.
The Captaincy General of Guatemala (Spanish: Capitanía General de Guatemala), also known as the Kingdom of Guatemala (Spanish: Reino de Guatemala), was an administrative division in Spanish America which covered much of Central America, including what are now Costa Rica, Nicaragua, Honduras, El Salvador, Guatemala, and the Mexican state of Chiapas.
In 1609 the area became a captaincy general, when the governor and Audiencia president was also granted the title of captain general to deal with foreign threats to the area from the Caribbean, granting the area autonomy in administrative and military matters.
The elite were the ones who united Central America with Mexico at Mexicos independence but that lasted only two years.
It should be said that Yucatan was also a Kingdom.
Captaincy General of Yucatán (Spanish: Capitanía General de Yucatán) was an administrative district of colonial Spain, created in 1617 to provide more autonomy for the Yucatán Peninsula, previously ruled directly by a simple governor under the jurisdiction of Audiencia of Mexico
The Kingdom of Yucatan included present day State of Campeche, Yucatan and Quintana Roo.
Mexico was called the Kingdom of Mexico.
So even Yucatan was not administered later on from Mexico City.
In above both a separate Intendancy of Yucatan and of Guatemala were created in the mid 1700s.
As for Guyana I would think a united Spanish America would have annexed much if not all of present day Guyana. Who knows they might have even annexed Surinam and French Guiana.
Even today Venezuela still claims 1/3 of Guyana.
Map: