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The leaves hanging on to your maple is likely related to an early cold spell, before the leaf abscission process was completed.

There is no harm to come of it, only an aesthetic issue.

Absolutely! This will be a good way to preserve the genetics of this tree. This article will help: https://www.gardeningknowhow.com/garden-how-to/propagation/layering/air-layering-plants.htm

As long as the graft appears to be taking properly, and there is no decay, then it will be fine. Most maples are aggressive growers, and adapt well to grafting.

This could, at one time, have been an infection. If the tree is still alive, then it will continue to grow just fine. If the infection seems to be present still, and the trees fail to thrive, then you might want to remove them.

If they continue to grow, then there is no harm in letting them live. Ultimately this will be up to you.

Follow a regular watering schedule during the first growing season to establish a deep, extensive root system. Watering can be reduced after establishment.
Feed before new growth begins in spring. Pruning time: late winter to early spring.
You will need a shade to part shade location and may need to provide shelter from wind.
You will need to add extra insulation in a container planting.
It's possible some of the roots have died off.

Check the branches; do they bend or snap off.
Also do a bark scratch test.
https://www.gardeningknowhow.com/ornamental/trees/tgen/tree-scratch-test.htm

If that container does not have a way to drain out extra water, then that will be the cause. Overwatering leads to fungal infections, and since they are both in the same container, the other one is likely not far behind to look the same way.

Make sure that these dry out very well before allowing extra water back in. Make sure that it doesn't get left in the rain or get watered when it is still wet.

Next you will have to go about treating the issue.

Apply a handful of DOLOMITIC lime, and WETTABLE sulfur to both containers to treat the sick one, and prevent the same from happening to the one that is still healthy.

You should start to see recovery within a few weeks of treatment.

This article will help you to care for these trees: https://www.gardeningknowhow.com/ornamental/trees/japanese-maple/japanese-maples-in-containers.htm