Can Jehovah's Witnesses hold and discuss openly with other Witnesses opinions that differ from orthodox WTS teachings? If no, why not?
No. The Watchtower strongly discourages Jehovah's Witnesses from private discussions of ideas not sanctioned by the Governing Body. The reason being is the tendency to form sects. As most are aware, the mainstream denominations of Christendom have splintered into hundreds of sects and sub-sects and cults. For the most part the Watchtower has been able to prevent that from happening among Jehovah's Witnesses.
It ought to be recognized that the apostles acted similarly. For example, Paul instructed Titus to be on guard against those men whom he described as "unruly men, profitless talkers, and deceivers of the mind, especially those men who adhere to the circumcision. It is necessary to shut the mouths of these, as these very men keep on subverting entire households by teaching things they ought not for the sake of dishonest gain. "
So, it is the responsibility of those in leadership positions within Christ's congregation to protect people from profitless talkers and deceivers who pose the danger of overturning our faith. Ultimately, though, the men in positions of leadership will be judged as to whether they have been faithful to their master.