getmininginfo vs getblocktemplate
1) "getmininginfo" gives the difficulty right now, and "getblocktemplate" gives the difficulty for the next block


That is correct. getmininginfo gives the diffculty of the block at the current blockchain tip, and getblocktemplate gives the target for the block to be mined on top of the current tip, i.e. the next block.


  under the assumption that this block will be solved within the next second.


There is no such assumption. The difficulty doesn't change by the second, it only changes at the end of each difficulty adjustment interval. If you are fully synced, you will always know what the difficulty of the next block will be. The calculation is completely deterministic and any block that has your current tip as its previous block will have the exact same target.