Not sure how exact this is. Treg Battery meter (charger/alternator) shows up to 16v !! And is constantly at 14v when driving.
14V when driving is perfectly fine, because that's when your alternator is running, and is charging the battery. The battery meter scale going up to 16 Volts is not there, because charge voltage should ever go near that value under normal circumstances, but to allow you to see when it actually does - meaning when you alternator/voltage regulator has failed. Then you indeed need to stop driving the car and have it repaired, because it will kill your battery. Actually, anything above 14.5 Volts will kill any automotive battery in the long term (even a regular, lead-acid one), and 16 Volts is so much, that it would boil your battery fluid practically instantly, and maybe even cause the explosion of it.
Oem Battery is lead acid so is it safe to go AGM if the véhicule charging system was not designed for an AGM battery?
I don't think any car alternator will overcharge, unless of course the alternator/voltage regulator in them is defect. They're working with a safety limit, and never actually charge the battery to full and/or with full voltage, but with a safe margin below it (like 85-95% max), to avoid boiling the battery during the trip.
Because T1/T2 Touaregs have a voltage meter in their instrument clusters, you can always see in them what the the actual charging voltage is - and also, that it never goes anywhere above 14.4 Volts (in a healthy car), but stays around or just slightly over 14V when charging. So, I'd say it's pretty safe to use AGM batteries with Touaregs - or with any modern car with a healthy alternator/regulator, for that matter.
Of course owner manuals should always be consulted regarding the charging voltage values/limits, but I doubt you could find a modern mass produced car in the 1st world markets that would charge its 12 Volts battery with anything above 14.4 max.