i was using my Full-HD monitor for this yesterday, so it was great to have more than half a dozen items side by side (i think I got up to about 8). but, you've got a nice slider there, so it appears to cope gracefully with longer list comparisons. i really hate websites that let you compare multiple products, but then limit you to 3 or 4 or 5.
q1: without reference to any specific category, it's nice if you can diff less than about 10-12 items, otherwise it can sometimes become mentally counter-productive compared to better tuning your search criteria first. BUT, it also depends on how well versed you are in a particular product category to begin with, as to how well you can set your initial criterial. sometimes using a 'diff' function like this can be a helpful learning tool to understand where you're swimming within a section of the product-category's spectrum.
q2: that's a tough one, because it depends where you're coming from: green-fields design, re-design, re-sourcing for cost minimisation, or re-sourcing alternatives that are 'close enough' to an obsolete part, and perhaps other scenarios? in each of these scenarios, some parameters are 'don't care', some are 'within a range', and others are absolute/identical, but all scenarios have a lot of overlap. here's a counter-question: would it be possible (practical, both technically, & at the UI level) to offer the ability to select which parameters to show in the diff? I think, for parameters that are binary & the user has specified them, exclude them from the diff parameters shown. for parameters with a range & where the user has specified their range, you need to show those parameters. but then you're left with all the parameters the user has't specified, & you don't know for what reason they've not specified them. perhaps this is where you could be 'smart' & list all those unspecified-params & prod the user choose which ones matter to them, up to whatever parameter-list-limit is in the back of your mind here.
q3: yeah I noticed when I went to do a diff on a list, there was already an item it in i'd put in there days earlier, & of course it would make no sense at all to include it in a hypothetical diff. my automatic response would be Computer Says Nooooo - you can't diff across multiple categories. perhaps offer a check-box on each item of the list, so the user can say, "ok, within this list, I see I have 3 resistors & 5 mosfets & 4 op-amps. tick the 4 op-amps & now give me a diff on them"? perhaps this is a half-way point to the cross-category diff within a list idea? but, i can see there might be corner-cases where you do want to diff across categories, though I'm wondering how useful that is in the grand scheme of things?