The passphrase set during the fundraiser can not be changed. After you have entered the fundraiser details, some wallets will store your private key in a keystore file encrypted with a password. In this case the password can always be changed. If there is no built-in function for this you can always enter your fundraiser details again and create a new ...
Flextesa's mini-net has an option --add-external-peer-port to allow other nodes.
It has been used to test 1 node joining the sandbox (incl. as a baker → in order to test Kiln).
I don't know of any attempt to make larger networks but it's worth a try.
The --peer= argument allows to specify peers that node should connect to. This is how you get private nodes to connect to specific peers. It takes an ip and port.
This is from running tezos-node run --help.
A peer to bootstrap the network from. Can be used several times to
add several peers.
As @utdrmac mentioned, it is not possible to reverse engineer the private key from the public key. Public Key is created from a Private Key using one-way functions (also called trapdoor) which is easy to do but the reverse process of finding a Private Key from a Public Key is practically impossible. Sharing a few links below to give you a better idea about ...
No, it is not. Public keys are derived from the private key. Imagine if I had your public key (which is public information) and all I needed was your password to gain access to all of your funds. That would be extremely insecure. A simple brute-force password guessing system could handle that in a couple weeks, depending on how complex your password is.
The incoming connection rejected due to untrusted source is most certainly due to a previous non-private run on the same IP. Your IP is known as a potential peer, so other nodes try to connect. It is not a problem, your node will simply reject these connections.
Maybe your public node was saturated.
It is important that your public node also has your ...
The passphrase used for fundraiser wallets contribute to the makeup of the secret key, so it's impossible to change it (these passwords aren't actually used for encryption). If you believe your wallet details may have been compromised, you should create a new key and transfer your XTZ.