Would disputing a delinquent account held in collections affect the date of last activity and alarm the collection agency that is withholding the account?

Nothing can legally affect the date of last activity. That date is established according to guidelines set out in the Fair Credit Reporting Act. When a collection account or charge off is reported to the credit bureaus, the data furnisher has 90 days to identity the DLA. It is set, established and cannot be changed. To do so is called "re-aging" and is a violation of the FCRA. If you send a dispute to the credit bureaus, their procedure is to contact the data furnisher and request "verification". This most definitely notifies the collection agency about your dispute. Who knows if they are "alarmed" by this. The bureaus have 30 days (from the date they receive your written dispute) to get "verification" or they must shield the account from view.