Hi Frans
I don't have this set so take my advice with a grain of salt.
Researching on the topic I found the exact same question as yours asked in the German Stummiforum
https://www.stummiforum.de/t1628...M-rklin-Erg-nzungen.html
If I understand correctly, the ICE starterset differs from the regular ICEs with a different and simplified electrical connection.
The item 44322 in the Start-up line is an extension of item 29320 (2012-2014) while item 39712 (2014-2025) is a four unit set (previous unit 29320 was a 3 unit set).

Your question: is a 7 car train possible?
The answer is about traction power and since there are no sliders in middle cars, the answer is probably yes with "gentle" slopes (3% or less) and clean track

Just my two cents worth
Jean

AFAIK the 36712 has already a 4 axle drive with 2 traction tire on the rear bogie so with clean wheels and rails and plastic cars it will be OK.
This is a compromise by Märklin to provide good current pick-up and reliable occupation detection already by the front bogie.
Just, periodically clean the wheels and tires with a cotton bud and Isopropyl alcohol
Here is the exploded view of the unit you consider purchasing (with two traction bogies and a central motor

To increase the traction power (not sure you need it) you may order a rear drive bogie E108 048 (30€) but flagged in yellow (not immediately available so better call Märklin shop)
https://www.maerklinshop.de/search?sSearch=E108+048+

Cheers
Jean

It's an affordable start set which has no red tail/marker nor interior lighting. Only reversing headlights, always on in cab control car???
The feature description doesn't say if the train can be expanded with additional coaches. There have been a couple of different coupler designs so only a certain type of add on car will connect to it.

You are sadly correct. Märklin has removed some part to lower their cost.
Must see if in reality the motor is fitted
One wild gamble is your idea: how about adding a supplemental rear bogie to the front with the following issues to adress
- negociate with M to obtain a supplemental rear bogie
- get a new transmission shaft or make one I think both transmission shafts are identical. Märklin describes them as Kardanwelle E216 777 €7.50 for two shafts and in stock
- adatpt a slider into the extra rear bogie. Chances are the same bogie frame is used on both the front and rear ones so only a matter of moving or adapting a small contact plate to collect power from the slider
- make sure the existing motor is fitted with a transmission joint on both sides

An interesting project

Cheers
Jean