I have some Time machine backups.
1 Are my documents as pictures etc saved there?
2 If I do a clean install will my documents on the boot hard disk be erased?
3 If I do a clean install will my wifi password etc be erased?
4 If I restore the Time machine copy will it not simply overwrite the clean install?
5 Will I need to reinstall my apps?
1- your documents and pictures should be saved in your Time Machine Backup. You can easily check by launching TM.
2- If you do a regular install (by running a USB installer directly over current OS) your files should stay in tact. (this is good) This can only be done with current or newer OS) However, if you wish to go to a previous OS, OR delete all files and start from a fresh and empty OS (clean install), run Disk Utility through the USB installer
3- let's assume "clean install" means wipe hard drive clean. all data including wifi password, accounts, apps, files, will be erased.
4 If you restore using Time Machine, it will overwrite with the Time Machine backup.
5 Will you need to reinstall apps? that depends what method you choose. If you do clean install, you start with nothing (which may be good or bad depending on you) If you restore from Time Machine, you will have your Apps or settings you had since the latest TM backup)