Ireland you literally have to show Proof the Credit Card covers Ireland, and most dont, due to the narrow roads, no shoulders, and right hand drive...i suspect they replace a lot of tires, rims, mirrors, bumpers and fenders on the left side...lol

So, the rental from Hertz for a Manual Diesel VW Golf hatch was $110 for 10 days, the Liability Insurance was $8 day...so with fees and taxes the rental as $230....not bad, but if you had to buy insurance on expedia at booking, thats another $110...

Sorry but the OP asked about which is best telco / sim card - they did not ask what type of map they should use - so any comments not about relevant sim card are off task

It's like someone asking how do I take the train from X to y and somebody suggesting a plane may be a better choice. It's relevant to the discussion. Informing the op about all the options and letting them choose seems like the best option.

The only thing you need is your ID card or passport to buy a SIM card. In FRA is a shop (O2)at arrival level in terminal B

I would suggest a SIM card with a cheap plan If you plan to use it your from time to time. But actually if it is a cheap phone you won't be able to use a lot of internet. On the other side If you want to use internet a lot you can go for a Pocket wifi for Philippines, it is generally a bit more expensive, but considering the fact that you need to buy a phone it might be the same cost at the end

Banks with ATM's that accept international cards are found all over China. A major city like Changchun will have numerous such ATM's. As above, you'll need to make sure that the Debit card is on a suitable international network (like VISA) so that it can be used in China. And contact the bank, and let them know that your son and his bank card will be in China.

Make sure he has some emergency cash with him, just in case. Money can be exchanged at major branches of certain banks, which are set up for foreign exchange. Probably in the center of Changchun somewhere, but I have no experience.

He should be paying for services in cash, and not trying to use his British debit card.

It's actually not a bad idea to print some extra passport photos to take with you to India while you are at it . They come in handy for the various ...such as if you intend to buy a Sim card you will need to supply a photo for that with the paperwork ..

Drive to Hoover Dam then up to Kaibab/Zion

You could visit Hoover Dam on one of your days in LV.

Agreed.

then up to Kaibab/Zion

Kaibab? I think you may be confusing some names here.
Kaibab National Forest is essentially the north rim of the Grand Canyon. That's well past Zion.
Kanab is a town in Utah, but it's still about an hour past Zion. If you're planning to stay here, you driving out of your way, then back again, losing time for little-to-no reason.
Kaibab, the very small town, is essentially Kanab.
Try to stay in Springdale for Zion.

Angel's landing trail and/or Observation Point Trail (I don't think I can do both?)

Angels Landing is actually not that long. If you start early, you could do both. Or better, you can hike Angels landing on the 7th, then hike Observation Point on the 8th.

Vermillion Cliffs, Drive to Page, Marble Canyon, Horsehoe Bend

A little bit of geography:
The Vermilion Cliffs and Marble Canyon are on US 89A. Horseshoe Bend is near Page, which is off US 89. There is a detour in order to do both, and it certainly won't happen in the order you indicate.
Here's a map.

Dec 10, 11: Antelope, Upper Antelope - Elopement???

Even with your plans to elope, I think this is a day too many. Do your elopement, then move along. Take that free day for Sedona.

Drive to Grand Canyon - South Kaibab hike

Since you're coming from the east, make a stop at Desert View Watchtower. It's the only place you'll see the river from the rim.

Grand Canyon (North Rim? South Rim?)

In order to get to Vermilion Cliffs, you drive by the entrance to the north rim. You'd have some serious backtracking to get there.
But then, the north rim closes for winter (with a gate), so it's not an option anyway.

You already have a hike planned on the South Kaibab Trail. That's squarely at the south rim. So you've already set which rim you're going to anyway.

For the Grand Canyon days, where is it best to stay? Flagstaff?

At the canyon itself.
But if not, Tusayan, which is just outside the south entrance.

Skipping: Valley of Fire

If you change when you see the Hoover Dam, as suggested above, you could easily replace it with Valley of FIre. It's on the way to Zion.

To attempt to do both within your time constraints is like a dog endlessly chasing it's tail i.e. it will drive you nuts.

I agree with #8 you do not need a 4 X 4. I have driven the length of the Carretera Austral since 1990 and current conditions are the best they have ever been. Many parts of the carretera have been paved or are being paved. Unless road crews are working on a particular section all of the Carretera is doable with a low clearance vehicle. During my last visit in 2017, the Puyuhuapi to Queulat section was quite rough due to road work which lifted huge masses of mud and dirt and which required careful maneuvering in a 2 x 2. It should be OK in 2019. In 2017 I rented a vehicle at PMC (Puerto Montt Airport) and BBA (Balmaceda Airport). BBA was certainly more expensive than Puerto Montt but nothing like the rates you indicated. I waived the CDL (collision damage waiver) at both, but considering that there is a lot of gravel pinging up from the road that could crack a windshield, you might want to add coverage. Have you tried Holiday Autos for car rental/hire?

Cell phone coverage. Surprisingly good. I am sure there are sections that might not have good coverage due to weather and geographical conditions, however my most recent experience was very positive, in that the cell phone towers are often placed near the Carretera itself, so you might be driving in the middle of nowhere (no people) and suddenly get pinging from the cell towers. I had good Wifi signal with an Entel Sim card even in Southern Tierra del Fuego usually before and after entering small hamlets. Same was true of the the Carretera Austral itself. Since there are small villages and hamlets spaced conveniently apart you won't have to wait too long to regain cell/Wifi coverage.

Coyhaique over Puerto Montt might be a better choice if you want to drive the Southern part of the Carretera Austral. Public Transport can be spotty especially out of the Summer months. It is somewhat reliable but not so much if you are keeping to a tight schedule and certainly much less useful if you want to link specific points. A rental car gives you that much more flexibility.

The Carretera Austral is a nature wonderland writ large. Overwhelmingly beautiful when the weather is good, still amazing and compelling when it is not. The San Pedro de Atacama area is interesting and beautiful in a completely different way to Patagonia. I have been to both, however unless you live in Norway/New Zealand/Canada and or Switzerland where beautiful alpine scenery is commonplace, Patagonia is something that you will remember for the rest of your life.