Half a shekel – half a shekel

Thank you very much Rabbi. I just wanted to know, 1) Where am I supposed to form a halakhic opinion on such a subject (which is only a memory of …)? And are there sources that can be discussed? Is this just a custom that began a few hundred years ago?
2) Perhaps since in all halakhic rabbis doubt rabbis' interpretation, then even more so in later custom. ?
3) Interesting to the point on the same matter: Does doubt from the Torah to the humara and doubt from rabbis' interpretation outweigh the custom of ruling as Sephardim or Ashkenazim (but then there would be no significance to ruling by evidence, because everyone would follow the rule of spikot)?

*Amendment B3)
Overrides “custom” to rule

I also see no way to formulate a position, so it is reasonable to follow the custom here.
There is no doubt that there is a custom.
This is not a question of which prevails but of the order of precedence. You apply the custom first, and then there is no doubt anyway, and there is no need to apply the rules of spikot. Just as when there is a presumption, you apply it first, and then there is no doubt. The laws of spikot are intended for a situation in which there is no other possibility of a decision.
This is the reason, for example, that in times of stress, they leniently consider an opinion that has not been ruled on by halakhah and do not follow the laws of spikot.
I think I dealt with this briefly in my article ‘What is Kula’.