1. BTC sent to my MyBitcoin account were sent to another address later (so it appears to me).
Did the owners of MyBitcoin do this?
The address is not "your" address it is an address in "MyBitcoin.com's" wallet that they use to track bitcoins received and they then [normally will] credit your account accordingly, after one block.
Those bitcoins received on that address are then spent during normal transaction activity within MyBitcoin -- regardless if you had any subsequent transactions to your account after the bitcoins were received.
A related post with some additional info.
[Edit]: I am going doing a test with an account on MyBitcoin.com
I just sent 0.01 BTC to 1CErUzvjoH8g2ripgofw7PHMLo6aCDonW3 which is an address I just generated for an account on MyBitcoin.com.
At the time I sent it, the latest block was 108,991. I will update status when we are at block 108,992. http://blockexplorer.com/address/1CErUzvjoH8g2ripgofw7PHMLo6aCDonW3
I'll confirm that after nine blocks confirmed, my transaction does not yet appear in the MyBitcoin account's balance. Withdrawals appear to be functioning fine yet.
[Update 2]: Appears that the problems with receiving credit for deposited BTCs at MyBitcoin have since been resolved.