Best way to tag every clickable element on the page (retail site)

I agree, that doesn't sound efficient... also, what happens as users mis-click, go back and change options, etc before continuing to the next step... that would be a lot of useless "dither" values that didn't account for anything.

There is no "one size fits all" solution, but for me, when the user chooses to continue, I would bundle all the selected information onto the single "continue" action... this would give me their actual selections, on a single server call.

I would also create a relationship, using s.products, between the product page, add to cart, checkout, success, etc....

On the product page, I would track the key info that I need at that level... the product identifier is the "glue" that will hold together all the touchpoints.

On the product page, I might have a few merchandising eVars relevant to that product and page, and I might also set up a custom numerical event for "product page". (I say this because I might use "prodView for all views of the product from product page, to recommended content, to other people also bought, etc...)

Then, when the user "Adds to Cart", I would use s.products with the same product identifier, the scAdd event, and again relevant merchandising eVars....

The Cart view, which may have multiple products, I would track all the products with the appropriate identifiers, the scView, and again relevant merch eVars....

The checkout, again, all the products would be tracked with their identifiers and merch eVars, and the scCheckout event

And of course on success, I would have the product identifiers and merch eVars, the Orders metric, and also the quantity and price values from the purchase.

In my reporting, I would use the product identifiers as my breakdown, and have the relevant columns: Product Page, Add to Cart, Cart Views, Checkouts and Orders.... 

Those values you spoke of may be standalone values associated to the entire order (payment options, in store/delivery, etc), or specific to each product making them part of your merchandising eVar content per product....

I know this is a bit vague, it's hard to provide specifics without seeing your pages / flows.

At a higher level, I'd also be asking whoever you're reporting to which are the metrics that matter to them in terms of business objectives and KPI targets, and tag those up first.

After that, it would be a case of looking at which elements are the most important in terms of understanding how users are using each stage of the checkout funnel, (do they get stuck entering their address, or is the add to cart page have too many options?) and then start to tag those to capture them as indicators of user behaviour.

The metrics that track user behaviour can then help explain the performance of the KPIs/ business critical numbers, and give avenues for optimisation

Well technically you could use ActivityMap to capture those link clicks automatically. What will be missing is any relation to actual products on the page. This will rather give you indication on where the users clicked on the page but not what happened after.