Instead on manually copying the eCart code back into the page, what happens if you apply the Add to cart button server behavior by going to Insert -> WebAssist -> eCart -> Add to Cart button?
for the DataAssist Error, try uninstalling data Assist, deleting the DW Cache file and then installing data assist again. When I check my copy of the configuration/shared/webassist/ApplicationBuilder/library/wa_configuration.js file, line 332 does not have a reference to a "extTtitle2 variable. In my copy, line 332 is:
var retArr = new Array();
this leads me to believe your copy is corrupted, reinstalling should fix the problem.