The guiding philosophy is to make it easy to do the right thing the majority of the time and to make features such as auto-complete be assistive and apparent, especially to new users. If we only did auto-complete on demand, as you suggest using Ctrl+Tab, then a lot of users might never even discover it. This is especially true with a feature such as extending comments or splitting strings. If you pay attention to the Feature Notifications on the status line, you can learn about SlickEdit features and also jump to links to tune them to your needs.
For example, extending comments and strings on ENTER is an example of doing the right thing in the easiest possible manner. If your cursor is within a block comment or a string and you hit enter, you don't want to leave it unterminated or malformed, so we put in the extra effort so you can continue typing the comment. Why make someone hit Ctrl+Enter to get the right result, and give them the wrong result the majority of the time otherwise?
If you don't like the default, it is easy to configure. Go to Tools > Options..., type "comment" in the search text. That's logical, right, you are looking for an option having to do with comments, that's how you would find it. Decide if you want to change it for all languages or just your programming language, and click on the "Comments" category which will be highlighted. In that option page, you will see options for Comment Editing, in particular "Split line comments" and "Extend line comments". Click on Help if you need more explanation about the settings.
As for Auto-compete, if you only want it to be on-demand, you can achieve that pretty easily. Go to the Auto-Complete options, and uncheck "Enable auto-completion". In most emulations, Auto-complete is bound to Ctrl+Space (codehelp_complete) and/or Alt+Dot (list-symbols) if you want to bring it up on-demand. You can use the Key Bindings dialog to change that to Ctrl+Tab, if you prefer a more acrobatic key combination.