A major principle of academic integrity is when in doubt, ask the professor. Academic integrity codes cannot cover all possible cases, and it's expected that all parties will act reasonably and request clarification when it is needed. You should not assume that "everything not explicitly forbidden is permitted", nor that "it is easier to ask forgiveness than permission." The answer to your second question is absolutely yes.
The case you describe in your first question is sufficiently questionable that I don't think you should proceed without explicitly checking. There is certainly the potential for this to be unfair to the Monday students who don't have the option of talking to anyone before taking the exam. If several of your friends consider that this may be cheating, that's a clear warning sign.
There are a few possibilities:
The professor hasn't thought of this issue.
The professor has thought of it, and doesn't want students to do it, but is afraid that mentioning it will only encourage students who wouldn't otherwise have thought of it.
The professor has thought of it, but thinks it goes without saying that this is inappropriate.
The professor has thought of it, and has taken other steps to ensure that students can discuss the exam without inequity (e.g. giving completely different exams).
In case 1, it would be inappropriate for you to take advantage of the professor's oversight. If you ask her about it, you might suggest that she make a policy and announce it explicitly.
Case 2 is perhaps a questionable decision on the professor's part, but in any case, by asking, you protect yourself from inadvertently running afoul of integrity rules. See also Case 3.
In Case 3, you also protect yourself by asking. Note that the university bodies that enforce academic integrity codes usually have no problem prosecuting students for doing things that they consider "obviously" inappropriate, even if no written rules explicitly forbid them. And since they are usually composed in large part of professors, they are likely to have a similar sense of what is "obvious" to your professor.
In case 4, when you ask, you'll be told to go right ahead.
My own practice in such cases is to write on the Monday exam "You may not discuss this exam with anyone, not even to say whether it was easy or hard, until Tuesday at 1:30". (In some cases I might weaken this to "anyone who has not yet taken it".) I do this even if the exams will be significantly different, partly because I feel that any differential in available information is a potential inequity, and partly so as not to tip my hand as to whether the exams will be similar or not. But as I said, you should not interpret the absence of such a warning as permission.