I do not see why a religion I make up 10 minutes ago should be any less of a religion than one made 100 years ago.
Are Catholics sincere if they have sex before marriage? Use birth control? Get a divorce?
Are Jews sincere if they eat non kosher food? Muslims if they eat non Halal and drink alcohol and gamble?
Christians who use Christ to persecute others? Such as gay people who want to get married?
The sincerity of the belief is a measured by the credibility of the claimant in each specific case. They are assumed to be credible by default unless the other party can show glaring inconsistencies. Even then, the guidance notes that people's beliefs change so challenging such a claim is relatively hard.
A sincerely held religious belief is basically a "get out of jail free" card (figuratively) because of the issues you point out. With the preponderance of evidence standard, all one has to do is convince the judge that their belief is more likely to be sincere than not. If one can prove that they go to church on a semi-regular basis, for example, it's nearly impossible to convince a judge that the belief is insincere.
It is not relatively hard, it is (or should be) impossible.
My religion stating that I believe in whatever I want to believe and is subject to change at any time is as valid as any other religion, from the perspective of the government. Assuming we are striving for separation of church and state.
Religion never needs to come up in legislation, just like one's favorite tv shows or music.