Very good input! I got so blinded with the card-sorting that I forgot to look into the alternatives. I am still not 100% familiar with all the different approaches yet, so I am glad you could nudge me in that direction.
Since Heuristic evaluation seems impossible to do because of the criteria for experts, I will be doing some guerilla usability tests, asking people in public to use the site on their smartphones. It seems obvious that people would want to order movie tickets on the go, and it is much less expensive/intrusive for me (as an individual) to conduct.
By platform assessment, all I meant was that I will be looking at- and mapping out the information architecture of the site.
I will be prototyping the site through paper prototypes first, and if there is time for it, a mockup prototype of higher fidelity, later.
I had intended to validate through looking at time-to-complete on a blocked set of tasks, tested iteratively through the stages, but I am not quite sure which other ways to validate. Is eliminating pain points a form of validation?
Thank you for your reply! That makes total sense, I had gotten so dead-set on using tree sort as a benchmark for later tests.