5) We have no idea when loadline will be out and what its features will be. Could be days, weeks, months, etc... so if you wanna download Wii U games now, go ahead. No one's stopping you.
4) You already mentioned in the thread that you're gonna get the gamepad.
3) A quick Google showed that Mario Party 10 comes with 5.3.2. Whether the exploit's exclusive to that firmware, or anything below, or whatever, refer to the first sentence of 5.
2) Another quick Google showed this thread:
https://gbatemp.net/threads/how-to-run-the-kernel-exploit-on-your-wiiu-tips.395335/. Since there are different payloads for different versions, the answer is yes.
1) Answered in the first reply of the thread.