2. A voter can only give money to a candidate who they can vote for and can only give money to a party they are a registered member of.
3. Organizations like corporations, unions, PACs, etc. can encourage voters to give money to particular candidates, but they can't collect money; only advertise and advocate.
4. At first there might be no limit to how much a particular voter can give to a particular candidate, but in future there might be some limit so that one very wealthy individual isn't allowed to "buy" a candidate.
Just because corporations, unions, PACs, etc. are "legal persons" doesn't mean they should have the same or more impact on elections than the citizens who actually can vote. These organizations should be limited to advocating for particular candidates via the mail. They should not be allowed to run ad campaigns on TV, Cable, or the internet because this is the current way these organizations outweigh the influence of individual citizen voters.
regarding your #2...
Some states do not require registration by party affiliation.