Your wallet will not be removed even after you remove the program. The wallet file is located in %APPDATA%\Sia-UI\sia\wallet. You can also locate the folder by going to the About tab in Sia-UI and press the button Show Sia Data. If you want to remove the current wallet from your computer, you can delete or rename wallet.db from that folder (Make sure to completely quit Sia-UI from the system tray icon at the bottom right before modifying the content of the folder). After that, if you relaunch Sia-UI, you should be given the option to create a new wallet or recover wallet from existing seed in the wallet tab.
I'm not familiar with antpool since I never used it before but i guess you need to edit the sub-account named "pay 12" and add your SC address to the Wallet Address section. BTW, I recommend you to switch to https://mining.luxor.tech because siamining have more than 51% of the network hashrate and could theoretically attack the network.
Well, since the transaction can be found on the explorer, the transaction is completed and the coins are now in a wallet poloniex controls. Their wallet are in maintenance right now. Sia devs had released the fix for their issue, but it depends on poloniex when will they apply the patch and re-enable their SC wallet. If you are not in a rush, I will wait them to re-enable their SC wallet and see if my coins appear. If not, the only way is to contact their support team. If you really need the SC now, you may contact poloniex support now and see if they can help you. Unfortunately, not much can be done except contacting poloniex since your coin is in a wallet they control.
Siamining.com I find to be the best option for the A3... since the second batch shipped the diff has doubled and I am making about half of what I was. I look for other pools to use for all of my Bitmain equipment. I don't like using antpool as I'd rather not give any extra money to bitmain after they flooded the market and screwed us all.