Alderdriver,
Look at it this way. If I deviate on a trip that goes from MEM to ATL and then later decide that I want to take the scheduled DH, can I un-deviate? We all know that the answer is no. Once you deviate, you are on your own.
The contract requires you to notify the company through VIPS of your deviation intentions. If you deviate on both legs, and then decide that you want to take the originally scheduled second leg, you can't un-deviate. When you check in on that deviation, VIPS says that you are confirming that you are within 100nm of XXX airport.
My "guess" is that the company looks at the language you quoted as you are deviating as you told us you would, and once you are in position to take the second leg that you told us through VIPS that you were going to take, you are now no longer deviating. Its just like when you do your final check in for a single leg deviation, once you are in position, you are no longer deviating.
I feel that stating that your interpretation is 100% correct and telling pilots that they can check both boxes when intending to take the company scheduled second leg is risky. I wouldn't want to advise anyone to do anything other than follow the proper deviation criteria, being more conservative if needed.
Are you 100% sure that you would win in arbitration if you checked both boxes in VIPS, and then changed your plans and decided to take the scheduled second leg and missed your trip because something happened on that second leg? To me, that is a pretty big gamble. If the duty officer or scheduler won't help you out, do you think the company will just pay you for the missed trip and let it slide.
It all boils down to you saying unequivocally that you are allowed to deviate one way, then change you mind and all is good. This is based on what you perceive as simple language. However, when other language is mentioned, you state that it is clouding the waters. Don't you think that both sides will use language to cloudy the waters in arbitration?
My advice would be to wait to deviate until you are sure of your plans. If you check the boxes indicating that you are going to deviate on all legs, then count on the company holding you to that. If you later end up on the scheduled leg and something happens and the duty officer helps you out, count yourself lucky. I see this as being more grey than black and white. YMMV, but wouldn't you feel terrible if a new hire followed your advice, missed a trip, and lost a job?