I have actually previously confirmed with ARC that United Blow can be used in conjunction with Riposting Blow and does not expend a spell, so if your riposte is a damage call you can add your highest damage United Blow option to it gratis. You cannot, as I was firmly told, add more than one spell to a single swing. So for me my Riposte can be 190 Body no problem, but 2100 Body is sadly not allowed. Ah well, Spellstrike Doom it is.