Hello @decentralization and thank you for your comment and for taking the time to read my post.

I really did two mistakes:

1. I thought sending funds to a steemit account required the use of the public memo key as an ID for which account to send the funds to, not realising that an account name only was sufficient (I thought the memo field was obligatory)
2. When making a transfer of 493 STEEM from Bittrex to Steemit I mistakenly copied the wrong key from the word file where I had saved the different keys, posting my main password instead of the public memo key (which was unnecessary in the first place) in the memo field and thus making it visible on the Blockchain (dunno if they saw it on my wallet, on Steemd, or just had a bot searching for it).

10 minutes later someone had spotted my error, changed all my keys, and sent the 493 STEEM I had just received to Blocktrades. At least I got my Account Recovery done very quickly, mostly thanks to a good community here helping me reach the right individuals, and knowing some myself that could help.

Anyhow, it was really awful, and I just hope I can help others not do the same grave mistake in the future.