ありがとうございます、海辺を歩くと心が落ち着きます。 質問していいですか、どうして第二かと第三かの文に[に]の代わりに[を]は使う選りましたか？

「〜の上を　歩く／走る／行く／通る」と言います。
「〜の上に　行く」とは言いますけど、これだと go onto の意味です。に means 'to'.

It could be 落ち葉の上に散歩をするとき／歩を進めるとき, but this に is used for で, which indicates a place where something happens. This に sounds softer than で. A literary expression.

I think in Japanese, を is to create something similar to the English direct object. And に is like an indirect object marker. We say 外に行く, 道（の上）を歩く, ネコにエサをあげる.
を indicates something that the verb's action directly affects.

すみません、「落ち葉の上に歩を進めるとき」は when you step onto fallen leaves の意味だと気づきました。Here に is also used as 'to'.

「落ち葉の上に散歩をするとき」This に is for で, but I don'r find such a usage on dictionary. Anyway, I do too think this way of using に is unconventional.

oh i see, thank you your explanation is very insightful. Would you say に for で is used to indicate defined concrete places instead of abstract expressions? Like e.g. being inside a house compared to floating "on" clouds

Yes, に is also used to indicate a place something exists like 人が家の中にいる or ものが棚の上にある. And we say 雲が空に浮かんでいる.
If it's 雲が空で浮かんでいる, this sounds strange like the cloud is intentionally floating in the sky.

i learned something new. Thank you :)

ありがとうございます