In the first group, sentence 1 is fine. Sentence 2 doesn't work because the verb is in the past tense, which conflicts with the adverb now.
In the second group, sentence 1 is in the past tense. It follows something like Why did you walk away yesterday? Sentence 2 is in the present conditional tense. It could follow something like If only you had a couple of dollars to spare!
Many grammarians today consider that English verbs have only two tenses, the present (simple) and the past (simple).
Some still consider the progressive/continuous and perfect forms as tenses, but most consider them to be aspects. It doesn't really matter to learners which word they use.
Very few grammarians consider forms constructed with modals to be tenses or aspects. It is much more helpful for learners not to think of future or conditional tenses in English. We have ways of expressing the future and conditions, often, but not necessarily, using modals.
I think that it is also worth mentioning that the past which the modal perfect refers to is relative to the time under consideration. In other words, it is similar in essence to looking back from a point in time. And the point in time may be in the future, present, or past:
1) Our boss has decided to call a meeting next week. We might have finished the work by then.
2) It's 7:30pm. He should have got there by now.
3) I wasn't feeling well. I must have eaten something bad.