[create-account]

Old Spanish was probably closer to the real pronunciation and had also more variety, depending on the whim of the scribe. Most of the greatest works ever written in Spanish are prior to the standardisation by the RAE at the end of the 18th century, indeed

[asveikau]

I don't know. An /i/ near a vowel in Spanish tends to become /j/. I usually see the consonant y transcribed as [ʝ].

Wikipedia on Spanish phonology notes the difference in pronunciation between viuda and ayuda which is an interesting contrast in two very similar sounding words.

Old Spanish had a lot of phonological differences so I'm not sure you can do a straight comparison.

[anthk]

Old Spanish it's something from 1200. Since Nebrija, the modern spelling was near already there.

[asveikau]

Old Spanish lasted until about 1500 afaik. The contrast wasn't just spelling, the spelling differences are due to deeper phonetic changes.

[anthk]

I know; but since Don Quixote and Lazarillo de Tormes the modern phonetic changes were already there.

I own an anotated Lazarillo book for modern Spanish readers, btw. It will describe you every ancient jargon and old sayings as footnotes. And, of course, some of the context.