The way I would do this, given the scripts from before is this:
First of all, add in the tracks to be incorrect answer responses... I am guessing that you want these to be contextual to the question, otherwise a single track for the incorrect answer would be OK (you simply point every incorrect button to the same clip). Set the end jumps for all of the tracks to be to select a question again (in the example, point them to the 'select_question' script). You can delete the 'Try_again' script now - it isn't needed if you want to simply go on to another question regardless of being right or wrong.
To keep track of the 15 questions (making sure none repeat), alter line 2 in the select_question script to be one more than the number of questions that you have got. Together with line 1 this counts how many times the script has run so currently on the 6th iteration you jump to the exit screen. You would change that value to be 16. You would also point the 'goto' command to whatever the last line in the script actually is... currently it is line 29.
Line 4 of the same script needs to have a modulus value of 16 as well, so that you get a possible 0-15 as values when you generate a random number.
You also need to extend the script to cover 15 questions, instead of just five. This is a case of copying the repeated sections enough times for each question. In the example project I have partitioned a GPRM (GPRM3, to be precise) into 16 bits and started naming each bit Q1, Q2, Q3 and so on. For each repeated section you would need to continue this pattern - rename the partitions to go up to Q15 (which leaves 1 bit unused) and make sure the repeated sections show the correct Q number.
You would add the new buttons to the menus, and create all 15 menus as you want them. Since you are not going to return to a question if you get it wrong, remove all prescripts from the menus. Currently these hold the values necessary to return to the last one played, so you can safely remove them.
However, you do need to extend the 'clear_all' script to cover every GPRM and partition and re-set them to zero
if you are going to allow the viewer to have a new attempt to go through your quiz. Currently, the single button on the 'Finished' menu points to this script. If you don't clear the GPRMs then you cannot do the quiz a second time without stopping and re-starting your disc/player.
In all, you can achieve what you want with two scripts - 'select_question' and 'clear_all'.
One word of caution - the select_question script is already using 68 commands and you may run out of space in that when you repeat the sections over and over. If you do, set the last command in the script to point to a new script which you will create. Call it something like 'select
questioncont' and simply carry on the script from there... so possibly you'll need three scripts. If you do use this splitting method then remember that line 2 of the 'select_question' script will need to read differently. You could simply set it to 'Jump Finished If (GPRM0>=16)' instead of having it go to the last line. You could also remove the last line if you do change line 2 since it is only used by line 2 in the script, but I'd leave it in as a catch all safety net.
This should be all you need to do to set this up, I think. If you do decide to keep a score then there will need to be another script or two (plus the possible scores as clips or graphics in tracks or menus - I would use a track), but it is all possible and could provide some useful feedback to your viewer. Good luck!