So, you've actually ended up asking multiple questions in one which strictly ([faq]) you shouldn't do, but I'll try to address it as one.
In relation to getting there in time for your flight, it depends on the airline you take, what they say, and also how much lee-way you want to give your arrival at the airport.
However, yes, you can take ground ...
As a certified diver (Are you?.. I guessed you are because you asked for diving not getting diving lessons) you usually know to find the information about dive sites in www.padi.com website firstly. Then you can search more by giving some key words to the search engines and complete it.
Dive Season: Year-round. Whale sharks sightings are ...
Passengers with a valid visa issued by a Schengen Member State, for a
maximum stay of 90 days.
International Air Transport Association
List of Schengen Member States.
Schengen Member States
Passengers with a valid visa issued by a Schengen Member State are
visa exempt for a maximum stay of 90 days.
You do not require a Visa for Belize but you do have to pay that $600 Fee
N.B. U.S. Citizens and Holders of U.S. Permanent Resident Cards/U.S. Multiple Entries Visas do not require a visa to enter Belize.
Source: Embassy of Belize in Washington, D.C.
Now onto the Fee part.
Visitors must hold at least ...
It is entirely possible to get a visa on arrival at the border, driving / crossing in from Guatemala.
However, note that for many countries, no visa is required for entrance. You can get a 90 day stay stamp basically without question.
For examples, stories, and pictures of the whole process, including some problems with border hopping, some bloggers have ...
There is a thread on Lonely Planet discussing exactly this topic. Which refers to 2 bus lines that offer service between Belize and Santa Helena/Flores/Tikal in Guatemala.
Express Buses – Flores / Belize / Chetumal
Two Guatemalan bus companies, San Juan Express Bus Service and Linea Dorada, provide bus service between Flores, Belize City and ...