For tactical reasons it would be helpful to know, if my opponent sees his cards in the same positions as I see his card backs. Can I rely on what hearthstone shows me, or is it just animation?
It's my opponents turn and he is thinking about playing a card, so I can see a card back highlighted. If he plays that card, is it the same he was looking at? If he doesn't play that card, but changes his mind and plays another card instead, and later in the game I see him playing the card which was highlighted before, is it the same card he was looking at some turns before?
The opponent is thinking and after some time he decides to obviously waste a card in order to give him a card draw. The new card is sorted to the right-most position in his hand (from my view). If he plays that card, is it the same, which he just drew? Or if he plays another card, will the card he drew remain in this position of his hand?
Knowing this would give some insight of his tactics and it would be possible to get some ideas what his deck relies on and which cards he is possibly waiting/expecting/hoping for to draw.
Depending on the answers it might be advisable to change oneselves behaviour when thinking about which card to play and not move the mouse above the card.
Yes, the cards in your opponents hand are in a fixed order. They are sorted by the time he/she drew them, with the leftmost card being the oldest. If you pay attention you can determine which turn the played card was drawn. There are a couple of tools that can help you with this such as Decktracker, which shows you which turn each card in your opponents hand was drawn.