There is an "independent action hypothesis" in virology. It states that each virus particle (virion) has an independent probability of starting the infection in the body.
So it it turns out true, there is no "safe level" - we could talk about an extremely small probability, but it would still be higher than 0 - not exactly a risk one would like to take.
Yes. Assume 1 virion has a 0.001 chance to start an infection. You could infect 1 person with 692 virions, resulting in a 50% change of them getting ill (1-0.999^692). Or you could infect 692 people each with 1 virion, with 50% chance of at least 1 of them getting ill (35% chance: 1 ill, 12% chance: 2 ill, 3% chance: 3 ill, ...).