It looks like there may be some confusion between JavaScript and Sawtooth Script. The former is able to grab information off the current page, while the latter is used to grab information off the server about other pages, lists, etc. If both of your questions are on this page, we'll only need to use JavaScript to read the current responses. Try to replace this:
[% "S1T1_" %] + i
with this:
SSI_GetValue('S1T1_' + i);
Additionally, your current for loop will iteration between i=0 and i=14. While zero-based counting is common in a lot of places in programming, Lighthouse Studio variables are one-based so we'll want these values to be one higher. Try this:
for (i = 1; i <= 15; i++) {
Finally, I noticed that you have mismatched brace characters. I would add a "}" between lines 9 and 10, and another after line 17.
Please give those changes a shot and we'll where that gets us.