Thanks for asking some in-depth questions.

1. Accounts are associated with a “seed" which is a key that allow you to send funds from a given account. Whoever holds the seed, controls the funds. This seed can be copied and stored under your bed, in a bank box, or wherever you want. If the seed is lost, the funds are locked. The actual account will forever exist on the public ledger which is copied and stored by the network nodes. This is similar to how Bitcoin works. See the docs for more info.
2. You need at least 0.1% of the total voting weight to be a representative node. The total number of representatives is limited to 1000 (which also means that the highest possible Nakamoto Coefficient is 1000). Possible attack vectors are described here.
3. You can create paper wallets and give them to your children. See this community project: https://nanopaperwallet.com/
4. This is related to the incentives for running a node. I answered this in the article. You can also read more in this article.