This is a good proposal for hand signals in 6.C.8., though the other submissions for replacing the word "doubt" should be considered to include.

Two concerns about the 6.C.7. proposal.
1) 6.C.7. primarily focuses on promptness of line calls. Would inclusion of "IN calls" in 6.C.7. imply requiring promptness for such calls?

2) The terms "call" and "ruling" have previously established meanings in the rule book. It would probably be best to preserve the consistency of their usages.

A line CALL is an OUT call (3.A.19.), (which may be followed by an "IN call").
RULING is synonymous with a DECISION, a final adjudicated call after input from partners, opponents, or referees.

6.C.5. "....If the opponents cannot make a clear “in” or “out” CALL, then the ball is RULED as being “in”.... ."
6.C.8. In doubles play, if one player CALLS the ball “out” and the partner CALLS it “in,”....... If the referee cannot make a clear call, the outcome of the opponent’s RULING will stand."

6.C.1. "....A CALL made by the opponent can be appealed to the referee for a final “in” or “out” DECISION."
6.C.5. "....If the referee cannot make a clear CALL, the outcome of the opponent’s RULING will stand...."
13.G.1.g. "Challenging the referee’s DECISION or RULING ...."
13.J. "When the referee’s RULING or DECISION is incorrect, the RULING will be reversed...."
Dictionary definition of RULING: "a DECISION of someone in authority, such as a judge."

A simpler solution might be to modify 6.C.8. only. "In doubles play, if one player calls or signals the ball “out” and the partner calls or signals it “in,” then ..... ."