Excuse my ignorance, but it is still not completely clear to me.

Fees are collected even if the songs sung have no known composer. I accept that few folk clubs/sessions could or would operate with such a restriction, but some might. If I formed The Society for Appreciation of Child's Ballads, would dues be required before anyone sang at a meeting? And those dues would go to boost the income of who? (Whom?)

Fees are paid to the songwriters/composers in proportion to sales, not performances. OK, I can see how it has to work that way, but sales stop after a certain time. How is the pooled money shared out? Is that based on total sales? Sales in any given year? Do performers have to keep rereleasing their songs in order to get paid for them?

How is the money split between songwriter and arranger?

How does it work with arrangements of traditional tunes? Do they have to be described as Trad/Arranged (performer on CD), so that the money goes to the performer, or who gets the money otherwise? This question arises from tales (perhaps misunderstood) told in other threads, of performer(s) receiving money from traditional rhymes, claimed by their arrangement.