What happens if Baron Rivendare and Dreadsteed die at the same time

Only those limited to one turn.

Mind Control and Mind Control Tech both take control of a Minion permanently. This is regarded by the game's rules engine as that Minion newly joining your army and thus follows the same attacking restrictions as a Minion you summon or play from you hand. That means, unless it has Charge, it can't attack that turn.

Shadow Madness takes control of a Minion for one turn only. This would simply not do much if the Minion didn't receive Charge implicitly. Actually, there has been quite a discussion about the decision not to write it on the card (supposedly to keep it simple), but it seems Shadow Madness is intended (and probably handled internally) to give Charge to the Minion.

From a logical point of view, this exception makes sense because the card would do very little without that mechanic (notably, it would work the same way it does now with Charge Minions, and might get a dangerous Taunt Minion out of the way). Seeing as this could theoretically be intentional behaviour, though, we could well see the card reworded in the future if and when cards are released that do exactly this. For the time being, it's safe to assume that cards which take control of a Minion only for one turn give that Minion Charge implicitly.

The key concept at work here is that of Card Advantage. The basic premise is that cards are extremely valuable, and having access to more of them than your opponent is a strong advantage.

Perhaps counterintuitively, card advantage is more important than mana advantage (especially in Hearthstone where mana is pretty normalized), and card advantage is more important than life advantage (especially in the early game, when a few life points doesn't affect anything). If you think about it, one minion card can be worth 10 life points if it gets some good attacks in. A spell card can also be worth a lot of life, if it kills another creature which can potentially wreck you.

Framing your examples using card advantage:

Elven Archer

• Option 1: Play on first turn, then get steamrolled by a 2/2 or killed by damage. Net effect: Gain 1-2 life advantage, but lose 1 card advantage.
• Option 2: Play when it can kill an X/1, then maybe even attack or block before dying. Net effect: Break even on cards (though note you may have sniped a far better card than your 1-drop), as well as maybe gain some life advantage.

Loot Hoarder

• Option 1: Burn the coin to play turn 1, then die to a Rogue. Net effect: Break even on cards (due to the deathrattle draw), 2 damage to the Rogue.
• Option 2: Save for later, try to block an X/2 or sneak in an attack. Net effect: potentially up one card (you lose the hoarder, draw one, but you killed their minion and/or made them burn a card) and some life. Worst case is the same as option 1. And you keep the coin!

Now of course there are exceptions, and there are aggro decks that eschew long-term card advantage in favor of agression. But the theory there is they are setting up a "ticking clock" that the opponent is forced to deal with, probably by making non-ideal decisions as far as his/her card advantage.

In general, the main idea is: Cards are powerful resources, far more than a few points of mana or damage. Treat them as a scarce currency, and spend them wisely to get as much bang for your buck as possible.