You are correct in saying that if the aircraft is ASDA limited you cannot increase V1 to accommodate an increase in mass. This is because the increase in mass and the increase in V1 would both increase the ASD.
But if the aircraft is limited by its ability to get to V2 and screen height within the TODA, then increasing V1 will reduce the distance to reach screen height.
In using the CAP 698 you need to take care to read the small print. Para 2.1 in Section 4 MRJT1 page 7, states that "The field length used in the graph is based on minimum V1 being equal to VMCG". This is true throughout the CAP698, so the speeds in tables 4.8 and 4.9 are also based on minimum V1.
As a general rule for the exams, unless the question contains some additional restrictions such as "the aircraft is climb limited", then the effects listed in the CAP are the ones that they are looking for in the questions.
The thread below contains material on this subject.
http://www.pprune.org/professional-p...=increasing+V1