Buck converter or LDO with multiple voltage levels
Your concern #1 is valid.  You have 16.7V at 500mA meaning over 8W of dissipation in your LDO.  Not a realistic option.  

Use a buck converter for both applications. There's no concern about output stability in a properly designed converter.  You can get efficiency of high 80's low 90's meaning you have to deal with a few watts of dissipation with a 30W load.  

Proper thermal design should handle that without an issue.