Off the top of my head, I would answer as follows.
Firstly, I am not encouraging you to eat the Americium 241 source, but when I say that the exemption of the source (based on a level of 5 mico-curies activity), is based upon assessing all possible ways for you to be exposed to it, that would include you eating the source. That's shocking to say and I am by no means suggesting that anyone should do this. Still, someone has done a calculation to determine what radiation dose would be received by such a foolish act and they have determined it to be negligible. In such a scenario the source would most likely be expelled from the body along the digestive tract and pass harmlessly out.
Speaking about the "disintegration of the source", if we ignore any "break-up" of the source (recoil and all that), what it means to say that the source is radioactive means that every second it shoots out, in this case, alpha particles (helium nuclei) and thereby leaves behind less radioactivity. A 5 micro-Curie source will emit 200 000 alpha particles every second (to begin with). Do not be frightened by this large number as most of these alpha particles will get stopped by air molecules immediately around the source without exiting the chamber of the detector (assuming no holes in the chamber outer wall). Even if there is a hole or two in the chamber walls alpha particles do not carry a lot of energy and again will collide with millions of air molecules as they travel and will very soon lose all their energy. Americium-241 has a half-life of 432 years. What this means is, if we could count the number of alpha particles emitted per second, today, and get 200 000, then 432 years later, if we measured, we would get 100 000 alphas per second (half of the original amount). After another 432 years, we would get 50 000 and so on. I agree that there may be mechanisms for the casing of the detector to break-down. The question of the possibility of alpha-recoil I see as a pure physics question which I will leave to the physicists to answer. The point is, should there be breaks in the outer casing of the detector which allow the escape of source material (speaking here about small fragments of the source rather than emitted alpha particles as we have said that alpha particles do not travel very far in the air). If small fragments of the source material do escape (by breaking off the source due to age) and even if those fragments got into your eye, these fragments (dust) would continue to emit alpha particles in your eye. However, these alpha particles would not get deeper than the first tissue layer of the eye before being stopped. It is not the alpha particle that causes tissue damage leading to radiation damage, it is the ënergy carried by a moving alpha particle. A moving alpha particle may be thought of as a small moving bullet. However, the bullet very soon loses energy and stops moving. A bullet that is not moving cannot do any damage. Once the moving alpha particle has lost its energy, by colliding with the more dense tissue in the first layer of tissue of the eye, they lose their ability to cause any radiation damage to the eye or to any other part of the body. The "dead" alpha particles will then most likely be washed out of your eye by natural tears or be absorbed into your tissue harmlessly. So yes we are surrounded by natural radiation, and as a member of the public, you may be exposed to many different sources of radiation. For example, exposure to your smoke detector or a visit to the doctor resulting in an X-ray image being taken. According to international standards, as a member of the public, you are allowed a total radiation dose of 1 millie Sievert per year, from all sources, without it being considered harmful to you. As a Radiation Protection Specialist, I work with radioactivity almost daily. I wear a radiation monitor which records my daily dose. I am allowed a total of 20 milli Sieverts per year (20 x your dose) without it ever being considered harmful to me. In my 30 years of doing this job I have never ever gotten anywhere near to 20 millie Sieverts radiation dose in a single year.
I hope this is helpful and please feel free to ask more questions.
Paul