1) Custom.css can be found in themes/flashlight/css/custom.css - you can access it with ftp tools like filezilla. WordPress will not show the file in the theme editor.
2) Go to Flashlight > Contact & Social Stuff and select the contact page there. Maybe you need to create a new page before (you can name it, etc. as you like but you need to select it in the dropdown then.
3) Do you mean the copyright text? You can edit it in sidebar.php.
4) Yes - you can create a page which will be used as homepage. Add your slideshow images to this page. Then go to Flashlight > Theme Options and select this page as frontpage (you can also set a static front page - Settings > Reading).
Afterwards go to Flashlight > General Settings and upload your standard background image.
5) The user manual can be found in the zip file you downloaded from themeforest. Open up the file and search for the "documentation" folder. You'll find a index.html file in it which can be viewed in the browser.