Re-entering Germany, with transit in the US, with only a national ID card and another passport

Question

I am a dual German and Mexican citizen, and I currently live in Germany. It appears I may need to travel to Mexico without a valid German passport and I would appreciate any information on whether I will have a hard time getting back or not.

More specifically, my previous German passport expired earlier this year, and I neglected to renew it since I was busy at the time and I had no plans to leave Europe. However, my plans changed recently, and I will travel next week to Mexico for the holidays.
As soon as the plans changed I started the renewal of the German passport, with all the speed that the express option, a fair amount of begging, and a freshly minted PhD degree (for what it's worth) could muster. However, there is a chance that the new passport will not be ready before I fly to Mexico.

The way I see it, this should in principle be fine. I have a valid Mexican passport with a valid US visa on it, and a valid German national identity card.
Both passports have identical names (modulo single vs double surname conventions) and Germany is OK with me having the two passports.

• The flight out is direct from Germany to Mexico, and I should be able to travel exclusively on the Mexican passport (except of course for outgoing German immigration, which should not be a problem with both the ID card and the old passport).
• The flight back is via the USA, on a US airline. In principle, I should be able to travel exclusively on the Mexican passport, using the visa to transit through the US, and then show both the ID card and the old passport at German entry immigration.

However, there's a few things that worry me about that scheme.

• I'm unsure how much fuss the German entry immigration will raise to the ID card. I've been able to enter that way previously; however, that was only on flights from the UK, and the person at the passport office indicated that there may be differences there. I find this unlikely but I would welcome confirmation that this is possible.
• I'm wary that the airline will deny me boarding for the flight back if they become unconvinced that I will be allowed into Germany. How likely is the airline to take the ID card as proof that they can deposit me at the far end without a problem? What about the expired passport? I can in principle enter Europe as a Mexican tourist, but that might require e.g. a later ticket back to Mexico, which I do not have.
• I'm also wary that the US will deny me entry if for whatever reason they feel I'm a stay risk. Thinking back on previous US transits, I can't remember whether they asked to see the second passport. To what extent does "this person will be able to enter wherever they're going next" figure in the logic of US border officer?

Are these real potential pitfalls, or am I simply stressing a tad too much over flying without a full set of docs? What sort of documents can I bring with me that will help smooth over any rough edges?

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Edit: thanks all for your responses, they've got plenty of useful information. I've just heard from the Bürgeramt that the new passport is ready, so all of this is indeed just over-stressing.

Answer 1

I'm unsure how much fuss the German entry immigration will raise to the ID card. I've been able to enter that way previously; however, that was only on flights from the UK

If you have the new biometric ID card, just use the automated easyPass gates (with the photo page of the card facing up, but rotated upside down) if available at the German airport you're departing from, and you won't even speak to an officer.

Even if you aren't able to use easyPass, you have the right to exit Germany on your ID card, full stop. It is not the responsibility of the German border police whether you're let into your destination, but that of the airline. Speaking of the airline, use your Mexican passport when checking in online for the flight to the US, as your US visa is inside it

I'm wary that the airline will deny me boarding for the flight back if they become unconvinced that I will be allowed into Germany. How likely is the airline to take the ID card as proof that they can deposit me at the far end without a problem?

Airline staff in North America will likely be ignorant, but ask them to look up the info in the official database of Timatic, which says:

Passport exemptions: Nationals of Germany with a national ID card

Finally:

I'm also wary that the US will deny me entry if for whatever reason they feel I'm a stay risk. Thinking back on previous US transits, I can't remember whether they asked to see the second passport. To what extent does "this person will be able to enter wherever they're going next" figure in the logic of US border officer?

If you have to, simply tell the CBP this: you'll enter Germany on a German ID card, which is the German equivalent of the US passport card, and can be used at all air, sea and land ports of entry on the European continent instead of a passport.

am I simply stressing a tad too much over flying without a full set of docs?

In short, yes! Like I said, you may encounter ignorant staff trying to create a hard time, but if taking the above in mind you will be absolutely fine.

Show the following documents at the following steps:

Bag drop at German airport (if you have checked-in luggage): Mexican passport

Outbound German immigration: German ID card (whether using easyPass or being checked manually)

Boarding gate at German airport: Mexican passport

Inbound Mexican immigration: Mexican passport

Bag drop at Mexican airport (if you have checked-in luggage): Mexican passport. Also German ID card if asked for proof of return to Germany.

Inbound US immigration: Mexican passport. Also German ID card if they ask how you're getting to your next destination.

Bag drop at US airport (if you have checked-in luggage): Mexican passport and German ID card.

Security control at US airport: Mexican passport.

Boarding gate at US airport: Mexican passport.

Inbound German immigration: German ID card

Answer 2

The border officials in Germany should never question you presenting them a valid ID card in lieu of a passport. From you arriving at exit immigration, they cannot tell whether you are booked on a flight to London (which you can enter on a German ID card — at least until Brexit happens) or one to Mexico (where immigration may require an actual passport). Presenting a boarding pass is not required.

Similarly inbound, they cannot tell if you just arrived on a plane from Dublin or the US. In the former case, the ID card was enough, in the latter case it may not have been, but who cares; you’re trying to enter your own country with a valid form of ID for crossing your country’s border.