A very simple question:
During stress test, it will take quite a while before a transaction gets the first confirmation, which will depend on the priority (fee, age of unspent input etc).
However, after getting the first confirmation, can I assume that the tx will receive the 2nd, 3rd and subsequent confirmations in every 10 minutes??
Yes. A confirmation simply means that a transaction has been included in a block. Once it is included in one block (one confirmation), the rest of the confirmations mean that the block it is included in is X blocks from the most recent block. The only way to get those is for more blocks to be found. Of course, due to randomness, blocks are not produced every 10 minutes. On average it is 10 minutes, but it isn't always exactly 10 minutes.