Having trouble designing my preferred backup scheme
First of all grab Ola Hallengren's scripts to do the backups (http://ola.hallengren.com) and drop the maintainance plans. Ola's scripts have much better retention settings and will allow you to set a mark in hours how long to store the backups and know how to take the backups of the secondaries.

And for your questions:


Set up a small script in SQL Server agent or Windows task scheduler that copies a backup file to a secondary folder where you keep it longer than the others or do a mirrored backup to a secondary folder every six months. A small powershell script or xcopy command that runs every six months and copies the last backup file will do just fine
Backup log DatabaseName to disk ='NUL' will make a log  backup to the windows equivalent of /dev/null, no disk writes or space wasted. Nogood to restore from though.