[SalesForce] Convert date in JSON to Date from String

Here is an outline for 1/2/3 assuming you want to insert the data. If it's an update you'll need to query the existing versions of the objects:

HTTPResponse res = new Http().send(req);
if (res.getStatusCode() == 200) {
    Corporation__c corps = new Corporation__c[] {};
    MySQLJSONParser p = MySQLJSONParser.parse(res.getBody());
    for (MySQLJSONParser.Corporation c : p.Corporations) {
        corps.add(new Corporation__c(
                Title__c = c.title,
                ...
                ));
    }
    insert corps;
} else {
    throw new MyException('HTTP callout failed:'
            + '\nendPoint=' + req.getEndPoint()
            + '\nstatusCode=' + res.getStatusCode()
            + '\nstatus=' + res.getStatus()
            + '\nbody=' + res.getBody()
            );
}

On 4, yes you can present objects returned by the parse directly in your page if you want to: Visualforce can present Apex object data but you have to do more work such as outputting your own labels.

Because you are manually deserializing you do not need to put the "req" wrapper in your JSON. This is only required when Salesforce is doing the deserializing for you with the method signature so that it knows what top level variables to assign the values to.

Note that you do not necessarily need to deserialize into a wrapper class either: you can deserialize straight into your list of AMSAccts objects. Here is a small sample of what the JSON would look like in the two approaches:

Using Wrapper

JSON

{"AMSAccts": [{"accid": "001"}, {"accid": "002"} ... ]}

APEX

AMSAcctsWrper accParse = (AMSAcctsWrper)System.JSON.deserialize(req, AMSAcctsWrper.class);

Straight To List

JSON

[{"accid": "001"}, {"accid": "002"} ... ]

APEX

List<AMSAccts> accParse = (List<AMSAccts>)System.JSON.deserialize(req, List<AMSAccts>.class);

As you can see, when your JSON is an array you can just start it with an opening brace. The second approach assumes that you move AMSAccts to a top level class. If you left it in the wrapper you would need to reference it as AMSAcctsWrper.AMSAccts.

Hope this helps!