2. What are the penalties if the validator is offline for 2-5 days?
3. What's the rate of return difference between solo staking and pool staking?
I don't like that the validator needs to have power and be online 24/7. My power and internet is on 99.9% of the time but sometimes things happen that's out of your control. So this is one of the reasons I have not staked any ETH yet.
I still think that you should not go alone but if you want to, you could do that. However one thing is not up for discussion, if your electricity goes, if your internet goes, that's going to risk how much money you make from the staking. That is why I think even if you go solo, open up a VPS account, it is only 5 bucks per month or even a good one is 50 bucks a month, and you could be online there 7/24 forever without any issues at all, it would really help you a lot.