I'm wondering about the orthography of the old english edh ð. It is always drawn (lowercase) as a curved d with the line through it. But I'm wondering if it would be acceptable to just have it be a regular d with a line through it as in đ, or otherwise what other forms it could take (other than the uppercase Ð).
Yes according to Random House Unabridged Dictionary,
a letter in the form of a crossed d, written đ or ð, used in Old English writing to represent both voiced and unvoiced th and in modern Icelandic and in phonetic alphabets to represent voiced th.