Here's my two cents>
Re: obsolete, archaic words. If they can't be understood via the context they need to be modified (as little as possible) or the song as a whole loses it's meaning and will fall by the wayside. Handing out a folder of footnotes at a concert isn't practical. For recorded songs there is the booklet option to define strange words (Having recorded several Robert Burns songs- in Scots - we felt it was necessary to keep the language and provide translations) However some songs just can't be rescued. Many of the 17th century catches use words or phrases that were clever double-entendre when they were written but the non-vulgar meanings have been lost so the whole point of feigned innocence has been lost with the evolution of the language.

Re: Writing down original songs. I have revisited the original "proper" version of many of my songs and found that many changes creep in. Almost always for the better. Words and phrases that don't sing well or aren't easily understood tend to get refined with repeated performances. I have never felt hampered by written words or music - maybe because I'm a lousy mimic.