hey buddy, this is how card games have worked since the beginning.
the simplest explanation is that those games started as physical games and there is no good way to keep track of such things in your hand since usually you would use some type of a counter(dice, pebble, or a count) to denote the strenght/defense of a card when it is on the battlefield and you can't hold counters in your hand.
the second thing is that in most games you are some type of a wizard or a caster and the cards themselves are your spells so it is not really the card that goes on the battlefield but the result of the card being used.
the gravestone situation is kind a the same as turning something in a gravestone is basically recasting it. but because of that you can have cards that turn into gravestone and cards that make a creature invulnerable. and you can use this mechanic too, for instance if your opponent gives a creature -1/-1 you can gravestone it, and remove the -1/-1 🙂
The language explains exactly what is going on.
when a card is in your hand (in a "not-in-play" state), it is not a zombie, or plant, or anything but a card. The effect of a card will specifically say "add +1 damage to a zombie". When the card is bounced back to your hand, it is no longer a zombie, but a card (with its original description), so the effect no longer exist, because it is a card now. Same thing with the Yeti. Basically, when you replay a card, it summons a BRAND NEW unit, and not the same one.
The reason for tombstone is that tombstone removes your creature out of play (they should explain this in a tutorial), which means it is not a card, zombie, plants or anything, it literally doesn't exist in a game, and an object called "tombstone" replaces it. When it comes back, it is a NEW zombie/plant/w.e because the OLD ONE DOESN'T EXIST ANY MORE. That's how exile works, a new copy is summoned after tombstone is removed, and not the original copy.
Chimney sweeper - This is a pretty easy one. The language on the card says "when played", after it is in play, w.e you do, the +1/+1 will remain, because that is the bonus the zombie gets "WHEN PLAYED".
On Toxic imp - The langue says "give all imp deadly". When this creature dies, then the "give all imp deadly" no longer exists on the field, therefore, no imps have deadly. However, because the sweeper's effect is when played, sweeper gets the +1/+1 when it remains in play.
