Is saying, "May your mother be blessed with Allah's mercy, if Allah wills," when supplicating for the deceased considered a lack of resolve in the request?

There is no objection to attaching a supplication in the form of a statement, such as "May your mother be blessed with mercy," to the will of Allah. This is not considered a lack of resolve in supplication. Supplications in the form of statements, conditioned by the will of Allah, have been reported in the Sunnah (definition) of the Prophet, such as his saying to a sick person: "No harm, it is a purification, if Allah wills," and his saying after breaking the fast: "The thirst has gone, the veins are moistened, and the reward is confirmed, if Allah wills." It is the supplication in the form of a command that is prohibited from being attached to the will of Allah, but there is no harm in doing so for a supplication in the form of a statement.