Solidity – How to Handle Callable Functions Not in ABI or Source Code During Contract Development

You need to use ethClient now

I am binding an object called NewMaths...

import "github.com/ethereum/go-ethereum/ethclient"
...

func getClient() (client *ethclient.Client, err error) {
    endPoint := "/Users/daveappleton/Library/Ethereum/geth.ipc"
    client, err = ethclient.Dial(endPoint)
    return
}
...
client, err := getClient()
if err != nil {
    fmt.Println(err)
    return
}
nnm, err = NewNewMaths(common.HexToAddress(address), client) 
if err != nil {
    log.Fatalf("Failed to instantiate the NewMaths contract: %v", err)
}

ownerTx, err = bind.NewTransactor(strings.NewReader(mon_key), ",password")
if err != nil {
    log.Fatalf("Failed to create authorized transactor: %v", err)
}

Keeping it brief as you look as if you have most of it. Will answer more if you need it.

Yes, it's possible.

contract Called{
    uint public myuint;

    function set(uint _var) {
        myuint = _var;
    }
}

interface Called{
    function set(uint);
    function get() view returns (uint);
}

contract Caller {

    Called public called_address;

    function set_address(address _addy) {
        called_address = Called(_addy);
    }

    function set(uint256 _var) {
        called_address.set(_var);
    } 

    function set_call(address _called, uint256 _var) {
        require(_called.call(bytes4(sha3("set(uint256)")), _var));
    }
}

You have two different ways of doing it as shown above.