1. Why the earlier tx included with the current one?
2. Is this the reason of the error(s) I experienced?
1. Im not sure. Ive never seen this happen.
2. Im pretty sure it is. Until someone who knows things comes here to help you fix that (I dont know, lol), you could wait for your first tx gets confirmed to spend the coins. You could also try deleting Electrums entire folder, installing it again and reimporting your wallet (please make sure you have your seed/backup if you do this).
Additional question:
What happen if I sign a tx but do not broadcast?
Nothing happens. The moment you broadcast it, the transaction shows up in the nodes mempools around the network to get picked by a miner.
Until you do the broadcast, its like the transaction didnt happen yet. Think of this like writing a letter but not sending it through the mail.