Re. #1: You cannot change the subject id / group id prompt or make it collect more / different data. Subject and -- particularly -- group id are used for randomization and condition assignment purposes in most procedures. If you want to collect additional demographic data, you need to do that in your script(s) by setting up e.g. a <survey> / <surveypage>s with appropriate questions. See the "demographic survey" tutorial in the Inquisit documentation for an introduction to those features. A <surveypage> element is a special type of <trial>: You can run them either via <block> elements' /trials attributes or via a <survey> element's /pages. A <survey> is a special type of <block>: You run it via your <expt> element(s)' /blocks attribute just like any regular <block>.
Re. #2: You need to first define a global variable (cf. the <values> element) and/or <expression> that represents or calculates whatever you want to log ("confidence" has no universal meaning outside the specific context of whatever procedure you are aiming to implement). You can then log that additional information to the data file by specifiying the <data> element's /columns attribute accordingly:
<data>
/ columns = [date time subject group ... values.somevalue expressions.someexpression ...]
...
</data>
See the documentation for the <data> element for details. As for using <values> and <expressions>, see the language reference for the <values> and <expressions> elements. You'll also want to familiarize yourself with the topics on event attributes (/ontrialbegin, /ontrialend, etc.) as well as the operators and functions topics in the language reference. You can find countless examples by either searching the forum and/or studying scripts available via the test library at
https://www.millisecond.com/download/library/ .
Re. #3: See the "How to erase stimuli" topic in the documentation's how-to section. Suppose you have three stimuli a, b and c and want to present them at 500 ms intervals, with a blank screen in between:
<trial sometrial>
/ stimulustimes = [0 = a; 250=blank; 500 = b; 750 = blank; 1000 = c; ...]
...
</trial>
with
<shape blank>
/ shape = rectangle
/ size = (100%, 100%)
...
</shape>
If you haven't already done so, I'd recommend working through all of the tutorials included in the Inquisit documentation. Those cover the basic aspects of expressing procedures in Inquisit syntax and will familiarize you with the most important syntax constructs.