Layover/Connection

All other things being equal (which they probably are not), I'd pick Minneapolis.

JFK and Heathrow are huge, crowded, and often challenging airports that I prefer to avoid. It's been years since I flew through Detroit (at one point in my life I flew through DTW frequently) but my impressions of the airport were not great.

Other factors...

I generally prefer to get the long trans-Atlantic leg out of the way first, connecting in Europe; I also try to maximize the length of that long flight, rather than having multiple long flights (the longer that flight, the greater the chance to get some sleep). Then there are the lengths of the layovers -- I look for a "Goldilocks" layover: not so short it puts me at risk of missing the connection, but not so long that it greatly drags out the length of my trip (Note: everything changes if I'm not on a single ticket, as the potential consequences of mis-connecting increase greatly). Time of day may also be a factor. The specific aircraft may also be a factor (seat size, and thus your comfort, is not the same across all planes).

So, all other things are probably not equal, and I'd consider those. That said, I avoid JFK and Heathrow whenever I can.

Why not take the direct AA flight?

I found connecting in Detroit to be straightforward.

Tossup on the US layovers—schedule, price, aircraft types would factor. (Presuming DL/KLM means JFK would be fine. A fully airside connection.)

I’d definitely avoid LHR. For a list of reasons.

I would not have any problem connecting in Detroit, Minneapolis or JFK. Since your trip is next year, your flights will probably change before your trip. We always have flown non stop from US to LHR but this year, and perhaps next, things are different with LHR.

What time of year? If it’s in winter, for example, JFK, Minneapolis, and Detroit would not be optimal.