You can’t disable those because engine needs them, thats why after commenting them out you started to have unresolved symbols errors from HTTP module which use curl.
I think you trying too aggressive methods here just because library in plugin you technicly trying to nuke any use of OpenSSL in rest of the enigne. Insted of destroying one side you should make them cooperate or at least coexist. Is there really API breaking differences between OpenSSL in UE4 and your plugin? did you tried your library in plugin with OpenSSL in UE4? Did you tried upgrading library in UE4 to version that your plugin needs? did you maybe try to modify build scipts in library to rename other version of OpenSSL in plugin?