Geonic kapparot -- based on Adonis gardens / swinging nettles?

Elul is here, and with it, the approach of kapparot. Nowadays people use a chicken. In prior times, people also sometimes used a ram, though presumably did not swing it about their head. For a long time, people have argued about the legitimacy of this practice.

An oft-cited Rashi on Shabbat 81b discusses a Geonic practice:

האי פרפיסא - עציץ נקוב שזרעו בו ובתשובת הגאונים מצאתי שעושין חותלות מכפות תמרים וממלאין אותם עפר וזבל בהמה וכ"ב או ט"ו יום לפני ר"ה עושין כל אחד ואחד לשם כל קטן וקטנה שבבית וזורעים לתוכן פול המצרי או קיטנית וקורין לו פורפיסא וצומח ובערב ר"ה נוטל כל אחד שלו ומחזירו סביבות ראשו שבעה פעמים ואומר זה תחת זה וזה חליפתי וזה תמורתי ומשליכו לנהר:

"This porpisa - a perforated planter in which one plants. And in the teshuvot Hageonim I found that that they make palm-leaf baskets, fill them with dirt and animal manure, and 22 or 15 days before Rosh Hashana, they make each one for each male and female child in the house, plant in it beans or peas, and they call it porpisa, and it grows. Then, on Erev Rosh Hashana, each one takes his own, swings it around his head seven times, and says 'this is in place of this, this is by substitute, this is my replacement' and then casts it into the river."

The Jewish Encyclopedia makes a bold assumption that this is "obviously" a carry over of the pagan Adonis gardens.

Another and apparently an older practise in geonic times was that of planting beans or peas in palm-leaf baskets for each child in the house two or three weeks before the New-Year. Then on the day before New-Year the children would swing the baskets containing the ripened plants around their heads three times, saying, "This be in lieu of me; this be my substitute and my exchange," and would then throw them into the water (Rashi, Shab. 81b). This is obviously a survival of the pagan rite connected with the so-called "Adonis gardens," Ἀδώνιδος κῆποι = "niṭ'e na'amanim" (Isa. xvii. 10; see Marti's and other commentaries). In Solomon b. Adret's time the kapparot ceremony was performed for the youths only (see "Bet Yosef," l.c.). According to S. I. Curtiss, "Primitive Semitic Religion To-Day," p. 203, Chicago, 1902, the Moslems of the villages of the Syrian desert still sacrifice a cock for each new-born son and a hen for each daughter born.

Perhaps. I don't see this as so obvious.

Read here in Wikipedia about Adonis Gardens:

Women in Athens would plant "gardens of Adonis" quick-growing herbs that sprang up from seed and died. The Festival of Adonis was celebrated by women at midsummer by sowing fennel and lettuce, and grains of wheat and barley. The plants sprang up soon, and withered quickly, and women mourned for the death of the vegetation god.

I suppose that the time is approximately right, and the quick-growing herbs are right. But I am not sure about the rest. And we would expect a connection to the month of Tammuz more than Tishrei:

Adonis was certainly based in large part on Tammuz.^{[citation needed]} His name is Semitic, a variation on the word adon meaning "lord". Yet there is no trace of a Semitic deity directly connected with Adonis,^[14] and no trace in Semitic languages of any specific mythemesconnected with his Greek myth; both Greek and Near Eastern scholars have questioned the connection.^[15] The connection in practice is with Adonis' Mesopotamian counterpart,Tammuz:

Women sit by the gate weeping for Tammuz, or they offer incense to Baal on roof-tops and plant pleasant plants. These are the very features of the Adonis legend: which is celebrated on flat roof-tops on which sherds sown with quickly germinating green salading are placed, Adonis gardens... the climax is loud lamentation for the dead god.^[16]

I would also see a connection to the swinging of nettles as a Germanic cure, and using nettles as a replacement for one's person. To cite The Big Bad Book of Botany, pg 36:

Mythology around the prickly nettle plant abounds. The Vikings believed nettles were especially important to the god Thor, and that burning one in a fire could prevent lightning strikes. Germanic cultures used nettles in medicinal rituals, believing that sickness could be cured by grabbing the plant by the roots and waving it over a patient while reciting his (and his parents’) names. According to Greek mythology, these stinging plants arose after a watchful father transformed his beautiful daughter into a prickly plant to prevent the god Apollo from seducing her.