I went to el tunco and was actually surprised by how crazily touristy and expensive it was. In fairness it did have 24h armed security at the town gate.
Thankfully these are all places easily avoided if you don't want to worry about those sort of things.
As for travelling alone Wilko there is a very well trodden backpacker trail through all of these countries. Someone will always be going in the same direction as you.
The high murder rates...
I would say for sure go to guatemala, just get out of Guatemala City and go straight to Antigua and use that as a base for any other trips you want to take around the country. Flores in the north is a good base for doing day trips to Mayan ruins and stuff too. I would 100% do the Acatenango...
Travelled through there with my girlfriend a few months ago.
Go to these places:
-the cenotes in mexico, they go all down the eastern carribean side, amazing if you like diving
-caye caulker, belize - belize city is a shit hole but you only need to go there to get the boat to caye caulker...