Ether Transactions – Sending Ether on Testnet via Command Line with Specific Gas Costs

Second question first: pendingTransactions are transactions held in memory, while getBlock('pending').transactions are the transactions included in the block being mined.

A full node checks blocks and transactions validity before relaying them. However it doesn't have to be a miner. No mining, therefore no transactions in getBlock('pending').transactions.

As for the first question, etherscan is a node with a handy web interface. Transactions that are broadcast to it can be displayed as pending even though it's not mining. You don't mention how long you left the transaction pending, but eventually it should reach etherscan. I look quickly but couldn't find transactions broadcasting statistics. It may be linked to that.

Another limitation is the pending transaction pool of your peers. Remember the infamous ICOs clogging up the network. If your peers have a full queue they may just drop your transaction.

Both transactions used the same amount of gas (21000 - minimum for a transaction). The second transaction paid a higher price for the gas (30 Gwei vs 21 Gwei). Not sure about how MyEtherWallet works, but the educated guess is that getting transactions done quickly required a higher gas price at the time of transaction #2. Usually you can override the gas price, so if you're not in a rush, you can dial that down to as low as 1 Gwei and lower your transaction fee accordingly.