I think your strategy works. If it were me however, I would book two J one way tickets. When I do this dummy redemption online, it shows as requiring 155,000 points, which you have 240,000 miles now anyways so that should cover it. You could even book 1 J return now as that takes 77500 miles. If you follow my strategy, after the o/b leg, you'd still have 85,000 miles so that would cover your 1 J return.

Getting 70,000 miles will take some effort, but CC churn and/or transfer from another program (such as your SPG miles would work too - it shows you as SPG plat). Transfer in 20,000 mile increments and you'll get 5K for free, returning an effective 1.25:1 ratio. Good luck in your journeys! Transfer in 40K, get 10K for free = 50K. You'll only need to get another 20K points - which is 1 or 2 CC's.

PS worst case, buy a Y ticket back...if you're married, sit up front. If you're not, I guess you'll stay in Y