What’s the quickest (free) way to get keys in Ancient Egypt

Since they come in sets of three, the Bloomerang does quite well; it hits all 3 parts of the camel and then all 3 of the zombies after breaking the screens, and it does so twice. Bloomerangs are less effective if there is another zombie in the way though, since they will only hit up to 3 zombies before returning.

I haven't gotten very far so I can't speak for the other plants, but you'll have the Bloomerang before you start seeing the camel zombies, so they're a solid early game choice. Logically speaking, any plants that penetrate or deal area of effect damage should do similarly well; the risk with these zombies is if you only damage the front they can easily overwhelm light or modest defenses.

No you cannot.

These plants must be purchased:

• Snow Pea
• Torchwood
• Jalapeno
• Power Lily
• Squash
• Imitator

I've completed the entire game and have not unlocked these.

On top of that, there are also purchase only bonus powers. You will be able to find one of each upgrade through the game, but the second one must be purchased.

• Shovel Bonus
• Plant Food Bonus
• Sun Bonus
• Bonus Seed Slot