As three seconds of looking it up (probably in the same place you found those forms to begin with) shows:
PBS *pílˀnas < PIE *pl̥h₁nós > PGmc *fullaz
PBS *wilkás < PIE *wĺ̥kʷos > PGmc *wulfaz
PBS *źírˀna < PIE *ǵr̥h₂nóm > PGmc *kurną
PBS *śírˀnāˀ < PIE *ḱr̥h₂-néh₂, *ḱr̥h₂-nó-m > PGmc *hurną
So evidently the *i and *u are epenthetic ...
Firstly, this quotation:
The distinction between /l/, /n/ and /r/, on one hand, and palatal /lʲ/, /nʲ/ and /rʲ/, on the other, is not always indicated in writing. When it is, it is shown by a palatization diacritic over the letter: ⟨ л҄ ⟩ ⟨ н҄ ⟩ ⟨ р҄ ⟩.
In second hand, there is not only hard L becomes w/o/u....
No, it is not acceptable and it is never done. It used to be done before the changes that appeared gradually in the 15th century, inspired by a paper most likely written by Jan Hus around 1400.
Before that the orthography did indeed use these digraphs, although somewhat differently.
cz meant c, not č
chz meant č
zz meant s
ss meant š
in the older form. ...
The question would be better asked as “When did the OCS ЪИ become ЪІ and when did ЪІ become Ы?” The three variants were originally used interchangeably, but later Ы took over, the most obvious reasons being it had the simplest shape of the three and since the yers disappeared as sounds it became irrelevant which one to use, ЪІ or Ы.
The first dated Cyrillic ...
In the Old Church Slavonic language (OCS), the little yus Ѧ represented a nasalized front vowel, possibly [ɛ̃], and is traditionally transliterated as <ę>, while the little iotified yus Ѩ, as it is clearly seen from its name, is [j] + Ѧ, that is [jɛ̃], <ję> and is a ligature of I and Ѧ. The little iotified yus Ѩ was written in the beginning of ...