I'd much rather layover at LHR than have to deal with both FCO and CDG. Spend a bit of time in the lounge at LIN (not been to the lounge as I was only FO at the time, but it's in a nice location and LIN is a nice, smaller airport), check out of the Schengen zone to fly to LHR, chill in the SkyTeam lounge, and fly back to the US. LHR will be easy to transit simply because there aren't a ton of transit passengers at T4.

Another advantage to booking LIN-LHR-BOS-DTW is the free PM changes. If they open last minute J space at low on one of the LHR-DTW flights, you can cut out a connection. I guess you could do that with FCO-DTW, too, but if you're flying on one of the days that there are two LHR-DTW flights, your odds are better.