Explosion venting panels on a dust collector should be sealed so that there is no water entering the dust collector or dust escaping. If there is any type of premature opening due to pressure pulses/low vacuum in the dust collector, then an incorrect panel was used for the application.
You can certainly retro-fit the proper explosion venting panels to protect your dust collector. The vent area would need to be calculated so that the panel would open only when the pressure inside the dust collector exceeded the Pstat pressure, that is, the pressure at which the panel is designed to open in the event of a developing explosion (usually .1 barg).

Any enclosure containing a combustible material has to be vented/suppressed and isolated if there is any risk that a spark could cause an explosion. Without having full knowledge of the iris valve design, I can tell you that for isolation purposes, the valve has to be certified as an explosion proof design. The need for an explosion proof design is to prevent the spread of any combustion incident that may occur in the hopper.
Separately, I am concerned about your mention of nylon material – what you refer to as a “nylon covered hole”. Using nylon or anything similar, coupled with the flow of bulk material, can create a favorable environment for static electricity, the discharge of which could create a “spark” event or, in other words, the ignition source for a combustible event.

The short answer is, “Yes there is a chance” and it is difficult to quantify. NFPA specifically mentions dryers, including vacuum tray dryers, “should be designed, constructed, installed and monitored so that the required conditions of safety for operation of the air heater, the dryer, and the ventilation equipment are maintained.” NFPA 654 A.7.18.
With respect to dryers, there is a risk of accumulated dust on the dryer or in the dryer and there is always the risk of a spark igniting a dust cloud. Safety measures to be taken are the standard: provide effective venting and isolation systems to minimize the impact of the explosion and prevent the explosion from traveling to other parts of the process.

Good question. Technically, you are absolutely correct in terms of the potential for harmful effects. Much of the standard was developed based on the best expertise and knowledge base of an earlier day, when chemical suppression was pretty much the only option for enclosures located inside a building and venting to the outside wasn’t practical. You really can’t use suppression in this case because of the pressure buildup in the enclosure, should the suppression agent be triggered. And if you were to build your 8 cu ft enclosure to withstand the pressure to accommodate suppression as a protective measure, you may as well spend the extra few dollars to build a pressure vessel to withstand a combustible dust explosion.
Today, with flameless venting systems designed for indoor or outdoor venting, an 8 cu ft enclosure can easily be protected. What might prevent this volume requirement from being changed is the need to recognize that compliance with the standards depends on reasonableness and competing priorities might require a balance to gain acceptance in the business community.

It really depends on how the rupture disc system was designed. The system you are using is a double bursting disc unit that provides double the protection for production equipment working under extreme conditions, such as highly corrosive environments or environmentally harmful or highly toxic substances. The response of the pressures of the two discs depends on the operating conditions and the burst pressures of the discs. If the burst pressure of the second disc (further from the vessel) was designed to be higher than the first disc, then it should remain intact. Also, you should have independent signaling units on each rupture disc so you would know if the second disc did rupture. Of course, if the first disc should rupture, to maintain the integrity of the system and its built-in safeguards, you should replace it immediately

Not safely. Any spark generated by cutting or drilling may start a chain of events that could lead to an explosion. To be absolutely certain you are proceeding safely, I would recommend the silo be emptied and cleaned so that in any case the dust concentration remains below the lower explosion limit (LEL).

Generally, metal dusts should always be handled very carefully in any manufacturing process. Shredding metal, depending on the process and the kind of metal involved, could cause sparks, an ignition source. If this process involves a combustible metal dust, and if it is concurrent to other processes, it could cause a fire or an explosion. You are better off separating the process as you describe.
Other NFPA codes besides the one mentioned which could apply are NFPA 485 and NFPA 122.

Not necessarily. You really have a number of options. Depending on distances, Preds and such, you might be able to duct and vent the collector to the outside with an explosion panel if your collector is close to an exterior wall. But you need to pay attention to the Pred of the collector and you may need to strengthen your collector (you mention it is old!) due to potential back pressure that will build up. Another option is to safely vent your dust collector with a flameless vent, designed to capture both the flame and pressure of a combustible dust explosion, should one occur. Or, as you mentioned, you might decide you want to move the dust collector outside to a “safe” location, away from the facility and away from areas where people might walk or gather or vehicles might park or be located.