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 transportation, and depending on what your plans are it'd be a consideration. You can get buses directly from Flores to Belize, however, if you're in Tikal, there's this from tripadvisor:
Don David's is fantastic - we were there in January. They will be able
to arrange all your transport if you need them to. I have rarely met
such helpful people! The buses to Belize run from Flores / Santa Elena
but will pick up at El Cruce which is the main road junction about 20
minutes walk from Don Davids. Alternatively, Don Davids will arrange a
ride into Flores. The buses run direct to Belize City with just a stop
at the border. We used the service in the other direction with a
direct bus from Chetumal to Flores via Belize City. The only advantage
in going to Flores is that there is more choice of agents but they are
all selling tickets on the same bus services.
So you could ask your Tikal Inn if they can do the same, if not, wander over to Casa Don David's and ask them for help.
What about driving? Some kind person has detailed everything you could possible want to know if driving from Tikal to Belize (well they went the other way, but just reverse it).
This site (Tikal Park) lists a few flight connections that may be of use to you.
And someone else also believes Tikal Inn can help you with connections, and provides you with some shuttle websites as well:
You ask the Tikal Inn and transportation agencies what they'd charge for a private shuttle. Here are 2 transportation providers I’ve been pleased with in Guatemala for share shuttles, private transfers, and flight arrangements:
Adrenalina Tours: www.adrenalinatours.com
Rainbow Travel: www.rainbowtravelcenter.com
Finally, there is some discussion of bus routes between Flores and Belize, but be aware the discussion is from a few years back, and may not be up to date.
Conclusion - if you're up for an adventure, I'd go for the bus ride. Driving sounds like too much hassle. But if you have the time, public transport is always a fun time, or a good story. Much more interesting than just another flight.