1. It is forbidden to interfere with G-d's decree.
2. It is an obligation to improve oneself both spiritually and physically.
3. Seeking medical attention is optional.

This would only be true if we assume that it was a decree of G-d that one is sick. However the sickness may be a result of human free will (e.g. negligence), which is not controlled by G-d.

And even if the sickness was decreed by G-d, maybe the decree was conditional; i.e. X will recuperate if X seeks medical care, X will not recover if X does nothing. Hence X seeking medical care does not contravene any Divine decree.

1. It is forbidden to interfere with G-d's decree.
2. It is an obligation to improve oneself both spiritually and physically.
3. Seeking medical attention is optional.

Our purpose in the world is the improvement of the world and of the human race both physically and spiritually. This is accomplished by the exercise of our free will within the laws of nature and morality.

Thus it is obligatory to ensure our physical well-being via human efforts, just as we are obligated to ensure our spiritual well-being via human efforts.

1. It is forbidden to interfere with G-d's decree.
2. It is an obligation to improve oneself both spiritually and physically.
3. Seeking medical attention is optional.

This is not correct. Our souls and bodies have been entrusted to our care by the Creator, however they remain His possessions. Hence our decision not to care for them, or to willfully destroy them has no validity.