Addresses – How to Manually Derive Address from Public Key String Using web3.py

Your calculations are right, except there aren't exactly 2^256 private keys -- there are "FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFE BAAEDCE6 AF48A03B BFD25E8C D0364141" (this number is named N in the ETH source code, and is the order of the generator of the elliptic curve secp256k1, from which Ethereum key pairs are generated).

In answer to your question, yes, private keys mapping to the same address will both be able to spend the money in that address on a first come first served basis. They will create the same public key, which the ECDSA verification algorithm is performed against, and so signatures generated by either private key will verify against the same address!

In go-ethereum/accounts/key.go, we have a private key generated from S256 which is secp256k1's curve, meaning they will be less than N be default.

func newKeyFromECDSA(privateKeyECDSA *ecdsa.PrivateKey) *Key {
    id := uuid.NewRandom()
    key := &Key{
        Id:         id,
        Address:    crypto.PubkeyToAddress(privateKeyECDSA.PublicKey),
        PrivateKey: privateKeyECDSA,
    }
    return key
}

func newKey(rand io.Reader) (*Key, error) {
    privateKeyECDSA, err := ecdsa.GenerateKey(secp256k1.S256(), rand)
    if err != nil {
        return nil, err
    }
    return newKeyFromECDSA(privateKeyECDSA), nil
}

go-ethereum/crypto/secp256k1/secp256 also generates the key pairs in accordance with secp2656k1 albeit in a slightly different way :)

Cool illustrative explanation for what N really is and why 2 keys will be able to spend money from the same account

Imagine private keys are miles driven in your car, and public keys are number of miles on your car's odometer. If the odometer rolls over from 999,999 to 000,000, a person with miles driven = 1,000,001 will have the same 'public key', and hence the same address, as a person with miles driven = 1.

EC group arithmetic is surprisingly similar to this, but rather than 999,999, we have the seemingly arbitrary number given above! Due to this property, even with your different 'private keys', you will be able to create signatures that verify against the same public key, and hence spend ETH from the same address!!

Boring math explanation

Private keys are 256 bit numbers, and to calculate the public key from a private key you multiply by the generator, g, of the elliptic curve group. The generator in use is defined in the parameters of the secp256k1 libraries being used in ETH. It itself is also an elliptic curve point, and as elliptic curves are cyclic, there exists an n such that n.g = 1 (this is called the generator order).

With this equation we can see that if we had a private key k with k>n, we would have k.g = (k-n).g = k'.g, for a k' that is possibly someone else's private key! So we have keys generated at random modulo n, rather than 2^256.

As mentioned by @jaime in the comments you need the position of the mapping variable to find the relation between address and storage key.

You can find a code I have written to detect the position for a basic erc20 contract below (using web3.py):

import json
from web3.auto.infura import w3
from eth_utils import remove_0x_prefix, to_int, to_checksum_address, to_hex

# erc20 contract address
CONTRACT_ADDRESS = '0x89205A3A3b2A69De6Dbf7f01ED13B2108B2c43e7'
# address with non zero token balance
HOLDER_ADDRESS = '0xD1220A0cf47c7B9Be7A2E6BA89F429762e7b9aDb' 

def getStorageAtIndex(i):
    pos = str(i).rjust(64, '0')
    key = remove_0x_prefix(HOLDER_ADDRESS).rjust(64, '0').lower()
    storage_key = to_hex(w3.sha3(hexstr=key + pos))
    return to_int(w3.eth.getStorageAt(CONTRACT_ADDRESS, storage_key))

for i in range(0, 20):
    if getStorageAtIndex(i) != 0:
        print("position is {}".format(i))

Result:
>>> position is 5

Once you have the correct value of position you can fetch the values stored in contract easily.

pos = str(5).rjust(64, '0')
key = remove_0x_prefix(HOLDER_ADDRESS).rjust(64, '0').lower()
storage_key = to_hex(w3.sha3(hexstr=key + pos))
w3.eth.getStorageAt(CONTRACT_ADDRESS, storage_key)