I've done the opposite with the tri first and marathon 6 weeks later. I think it's really tough to get the most out of both races doing the marathon first. Recovery timeline will vary person to person, but I'm destroyed after a hard marathon. Soreness for me goes away in 3-5 days, but it takes me more like 2-3 weeks for my legs to start feeling "normal" again. I wouldn't count on gaining all that much fitness in the period in between the two races.

70.3/IM training can get you in really solid marathon shape alone. If it were me, I'd approach it like training for a 70.3 with longer long runs once you get within the last 2 months before the marathon. I'd try to keep 3x swim/bike for as long as possible, can lower the intensity on the bike as needed to compensate for the increased run load. Recover hard after the marathon and start getting some super easy swims/bike in that first week after. Then it's more about maintenance and getting some 70.3 race pace type workouts on the swim/bike in the last few weeks. Run fitness will be there for the 70.3, so less is more and would err on the side of recovery there.