Apigee user authentication, access_token, and cache login session in iOS

Question

I'm new to Apigee and can't seem to wrap my head around it. I am familiar with implementing an iOS app that talks to a database via a webservice call. The call would involve passing back and forth JSON or variables though POST, GET, etc.

The user flow I envision is a lot like Facebook long term token storage. Here are the steps:

1. Type username and password to login.
2. The app will remember the access_token in the keychain.
3. The access_token will be used with any future requests such as updating profile. This way the user doesn't have re-login every time he/she is using the app.
4. Log out will clear all the token.
5. If the token is invalid or expired, the app will take the user back to login.

I've taken multiple routes and ended up getting stuck on all of them when it comes to Apigee.

ROUTE 1

I made a call to logInUser and receive access_token in return.

    [self.apigeeDataClient logInUser:username password:password];

All this is good until I want to update user's email address using the code below.

    NSMutableDictionary *requestDict = [NSMutableDictionary dictionary];
    [requestDict setObject:email forKey:kDataEmail];

    NSString *url = [NSString stringWithFormat:@"%@/%@/%@/users/%@?access_token=%@", BASE_URL, UG_ORG_NAME, APP_NAME, [userData objectForKey:kDataUUID], self.accessToken];
    NSString *op = @"PUT";
    NSError *error;
    NSData *jsonData = [NSJSONSerialization dataWithJSONObject:[NSDictionary dictionaryWithDictionary:requestDict]
                                                       options:0
                                                         error:&error];
    [self.apigeeDataClient apiRequest:url operation:op data:[NSString stringWithUTF8String:[jsonData bytes]]];

It seems that every other time it's giving me "No content to map to Object due to end of input" error. I checked out this thread but have no luck. I made sure the JSON object is not null. I tried changing the operation from PUT to POST and GET. None of which update the email address in the database.

ROUTE 2

Instead of using apiRequest, I switched to updateEntity.

    NSMutableDictionary *requestDict = [NSMutableDictionary dictionary];
    [requestDict setObject:email forKey:kDataEmail];
    [requestDict setObject:kDataUsers forKey:kDataType];
    [requestDict setObject:self.accessToken forKey:kDataAccessToken];
    NSString *entityID = [userData objectForKey:kDataUUID];
    [self.apigeeDataClient updateEntity:entityID entity:requestDict];

It looks promising except I started getting "Subject does not have permission" like the issue described in this thread. I tried calling assignPermissions like mentioned in Apigee document but that didn't solve the problem. I even provide access_token with the call, even though I shouldn't have to.

In the attempt to avoid calling login also tried calling storeOAuth2TokensInKeychain and retrieveStoredOAuth2TokensFromKeychain mentioned here. That didn't work either.

The only thing way to resolve this error is by calling logInUser before making a call to updateEntity. This means the user will have to login every time he/she wants to use the app. I know I can store username/password in the keychain. But before I do that I'm wondering if there's better solution out there.

I know it's a long post. So thank you for reading this far. Any pointers are greatly appreciated.