No, it would not. --reindex only handles the data that is stored on disk. Blocks are stored in the order that they are received, which is what brought you to your current view of the blockchain. So all that would happen is that you end up at that same state.
Switching to another blockchain fork requires your node to be aware of it, so you need to have ...
Running a pruning node simply means enabling the prune setting.
In Bitcoin Core, that can be done either in the bitcoin.conf file by adding a line "prune=N" (where N is a number in megabytes), or through the Qt GUI in the settings screen.
Electrum is not a full node, and never downloads or verifies blocks at all, so there is nothing to prune.
There is no ...
1) Which secure full node wallet implementations exist? Currently I just know of bitcoin core. Is there not any other one with a GUI?
I don't know of any others, apart from forks of Bitcoin Core like Knots. See https://bitcoin.org for an overview of various types of wallet software.
2) Can you use a Ledger device with those full node wallets?