Attribution nightmare - absolutely. In my experience with HubSpot and Marketo across long sales cycles, trying to tie brand search directly to closed-won is a fool's errand unless you've got a rigorous offline CR attribution model.
What I've found works: decouple brand search volume from revenue entirely. Run it as a separate trend line against lead velocity and SQL conversion rates instead. If brand volume dips but conversion rates stay flat, that's a signal your competitors are eating your search share. Conversely, if brand volume climbs and SQLs lag, your nurture sequences might be leaking.
I'd also layer in Google Trends for your non-brand terms as a control - if those track with brand volume, you're probably seeing genuine awareness bleed, not just paid search stealing.