1) Why would you stop? If you rarely downvote otherwise, then you can downvote several spam in a row and have no risk of losing any XP. Then you can downvote another set of spam in a row and likely lose less than 1 XP for each 3 you downvote. So do you have a pathological devotion to never losing a single XP that you'd rather see spam stay longer just to avoid losing 1 or 2 XP?
The rules get adjusted because there are actions that the rules reward that are not beneficial. These are pretty simple rules so they will never be perfect. So, of course, there are beneficial actions that are not rewarded by the new rules. You are (still) expected to use your judgement. This is part of why the motivations and discouragements are fairly minor.
2) Up-voting a few nodes by the same author is not punished. At some future point, up-voting more than a few nodes by the same author might reduce the number of votes you get just the next day, not much to worry about. Use your judgement.