I've been in exactly this situation.
There are no official restrictions (see below), but you may get asked extra questions. As long as you weren't denied entry because you were in the US too long (and it sounds like you weren't), this is not a "bad" denial as if you were a criminal and denied entry.
I couldn't find any information on this, but I have had friends who were denied TN-1 entry (or forgot to return their TN-1 card on exiting) that still get questioned about it 10 years later.
No, not required, but each border agent is different. Officially, if you are going for a short period of time for business, no one cares: provided you are explicit it is for meetings, no work will be performed there, etc. But again, the "official" rules are subject to the whim of the border agent. The key is to prove that no work will be performed, otherwise it falls under the requirements for a TN-1.
The nonimmigrant NAFTA Professional (TN) visa allows citizens of Canada and Mexico, as NAFTA professionals, to work in the United States in prearranged business activities for U.S. or foreign employers (http://travel.state.gov/content/visas/english/employment/nafta.html)
It should not, as it is not a "real" denial of entry in any sort of criminal-related manner. The border agents in Ottawa were quite clear about this.
The most relevant sections are here:
For example, if the inspector denied the TN application due to an accumulation of unlawful presence under INA § 212 (a) (9), this could result in a 3/10 year bar to the U.S. Egregious immigration violators may be expedited removed from the U.S., which results in a 5 year bar to the U.S. If the border inspector denied the application based on fraud under INA § 212 (a) (6), this would result in a lifetime bar to the U.S. In most of these scenarios, only subsequent obtainment of a waiver may permit entry to the U.S.
(That doesn't seem to apply to you.)
If the inspector denies the TN visa application because he/she believes that the applicant does not meet the basic requirements for the TN, the denial (generally issued under INA § 212 (a) (7) (A) (i) (I)) normally does not result in a specific bar to the U.S. However, the denial usually will result in heightened scrutiny the next time the individual seeks admission to the U.S.
(This does seem relevant)