Second push of TOGA below 800ft will just change the thrust limit to full GA thrust, but not the thrust itself. Above 800ft, the same thing happens, but the thrust levers advanced to full GA thrust as well. Note that full GA thrust is not always 100% N1, but it defends on altitude/OAT, and there is a table for it in the FCOM PI chapter.
Manually firewalling the thrust levers at any time provides maximum rated thrust, which is slightly higher than the TO/GA limit, and can be used at any time without damaging the engines, provided the EECs are in normal mode.
You are correct about the windshear manuever, but the automatic go-around on the 737 is only available with a dual channel approach, which is typically only flown during Cat 2/3 operatons, and even then only once FLARE is displayed as armed on the FMA (typically from around 1000ft AGL down). Anywhere outside of these conditions, pressing TOGA will disconnect the autopilot and you should follow the standard windshear escape manuever for manual flight. Typically, autolands aren't recommended in windshear conditions, and windshear conditions are quite rare when low visibility prevails.