Hello!

The 'R' is used, as far as I know, at the end of the words.

Yes, it should be 'sialdan' for 'sjaldan'.

Yes, 'nn' is given as 'n'. 'Þisar:runar' or 'þisarunar', I believe both
can be seen, best use the latter one...

For 'æ' write following rune: 'I' as staff and a stroke through the
middle from downside left rising up to the right. Should be nearly the
half the length of the staff.

Please realize, that the language in the time of the younger Futhark
differed from what now is known as Old Icelandic. If you go into reading
runic inscriptions you´ll see, that the language was far less
standardized than what you can see in modern editions (this is true for
the manuscripts too).
Hence you´ll normally encounter some problems transferring normalized
Old Norse into runic inscriptions. But don´t worry there are lot´s of
failures in the inscriptions too. So your inscription might be as bad as
the original ones :-D ...

Greetings
Stephan

--
"Why? Why am I punished? I`ve led an innocent life ..."

"Thou art a child of nature.
Thou shalt know that it art a crime to murder thy mother.
Yet, I hear her skriek in horror ... "