My experience on agency-issued tickets, at least for the one I just booked via Cap1 travel, is that if you cancel the itinerary via UA.com, which can be done after 24 hours, is that it is treated just like any other ticket booked with UA.com. In fact, in my case, canceled a ticket for wife/daughter booked via Cap1, and used it to book their next trip no issues and no additional hassles.

My understanding is that the restriction on residual credits, which is indeed part of fare rules, needs to be enforced by a TA, but is superseded by UA policy once you change/cancel on the UA site. In other words, while the fare rule technically doesn't allow residual to be issued for any ticket, no matter who issues it including United itself, handling this via UA directly, when available, does not lead to the residual restriction. At least for tickets booked via Cap1 Travel, and I think Chase Travel as well, they can be changed, etc. without further intervention directly with UA. Have no idea about other major or smaller/independent agencies, as I have not used anything else for UA tickets in probably about a decade.