I've actually seen this same thing with a rasperry pi that I use vncviewer to connect to.  The problem didn't have to with the QT wallets rather just a general problem with using vncviewer.  I still hadn't had time to figure out what was wrong with it.

Sounds like maybe it's specific to the application so probably different then what I am seeing.

For online bitcoin wallets, most exchanges will use HD(BIP32) wallets, it will generic the extended key offline, store the correspondent public key on the server, and it only needs to allocate an index number to each user, then new addresses can be generated on the fly whenever user requested a new address.
By using HD wallets, it is much easier to manager, because there is only one private key. Also it is much simple to operate, because each user can have as many new addresses as they want, technically 2³²-1 addresses, and no private key is needed to generate new addresses.

My point exactly; you don't need a POS machine because you're dealing with cryptocurrencies.

Exactly, yes.
All the customer need is a crypto wallet.
All mobile wallets support QR codes so you just need to scan the QR code.

That being said: You really really shouldn't start an exchange. Judging from your questions -- don't get me wrong, it's great that you are asking and trying to learn -- building and running an exchange is way beyond your current capabilities.

POS machines connect to a centralised backend (usually bank clearing)
Cryptocurrencies are the exact opposite of that.

The only thing similar is payment processors like Coinbase or Bitpay, which is already what most merchants use.

You'll scan a QR code, and send bitcoins to that address for your purchase, and in the backend, either the bitcoins go directly to the merchant's address, or through the payment processor and they get settled in fiat.

You'll need a standard database to get started with (e.g. PostgresSQL/MySQL).

This is needed to track open/close orders as well as the private keys associated with each user account (private keys are randomly generated).

Then to verify the balance of those user accounts, you can either use a full node or communicate with an API (e.g. blockchain.info)

Cold storage for an exchange wallet should be where you start. Then have some small amount that is not cold.  

And bitcond on the web server?  That is a bad idea.

The fact that you are asking these questions is quite concerning for anyone who would want to use this exchange.  My suggestion is to do a lot more reading to see how exchanges have been hacked, what others have tried and recommend and then think about the architecture you want to use based on that.

Use wallet software.

Wallet software generates the private key and stores it in the wallet file for you.

Some of the wallet software also allows import of private keys that were generated in other wallets.  In that case, you need to learn how to use the import functionality of the wallet software that you choose.

Can not enter online wallet, bug, fix please!
Error: Username, password or 2 factor is incorrect ---- I fill everything correctly, this is a wallet glitch!

Because exchange core called core for a reason. You can't let someone who you can't reach to code most critical part of your whole business. The core is the only thing that have direct access to your user ballance sheet, the subsystem that finalizes trades. You really want to keep the person in charge of core as close to you as you possibly can and ideally have at least 2 people working on it. Make no mistake - amateur can't do code review behind those people, and if they smart enough to build a core, they are smart enough to come back in a year and drain your wallets empty.

I have a file.  I put a secret (secret_A) in the file.  I give you a copy of the file.  I add a new secret (secret_B) to my copy of the file. I encrypt my file with my password.  You add a new secret (secret_C) to your file. You encrypt your file with your password.  Who can access the secrets?