Back to linguistics, it's beneficial to explain why their "issues", or what I like to call, "turds", are inane so that no sensible reader could be confused by my judgement calls. The first turd was their imaginary "language universal" such that relative pronouns are always declined according to their role in the subordinate clause and never by their role in the antecedent. The second turd was the general ignorance they had regarding Etruscan's relative pronoun ipa (and about the language as a whole, for that matter).
Concerning these imaginary universal case-agreement rules
Speaking globally, the choice of a relative pronoun's case is *not* necessarily bound to the subordinate clause alone, despite the persistent shouting from this normally silent group of persons. For example, on the grammar of Old English, Gotti/Dossena/Dury, English Historical Linguistics 2006: Syntax and morphology, v.1 (2008), p.11 explains that a relative pronoun may share with its antecedent "features for number and gender, and, optionally, for case". An example is:
Ic wat witodlice ðæt ge secað ðone hælend ðone ðe on rode ahangen wæs.If a relative pronoun was only about its relative clause, we should expect "who" to be declined in the nominative since it's the patientive subject of the participle formation, "was hung". Classical Greek also throws a curve now and then, and same too for Arabic where the dual relative pronoun agrees with its antecedent in gender, number and *case*.
"I know truly that you seek the Lord (ACC.), who (ACC.) was hung on the cross."
Regarding Ancient Ugaritic, Roger Woodard publishes in The Ancient Languages of Syria-Palestine and Arabia (2008), p.31: "The relative pronoun agrees in gender and number with its antecedent; whether the case of the relative pronoun itself is decided by the case of the antecedent or by the function of the relative pronoun in its clause cannot be determined [...]" Even Woodard is unaware of this alleged "language universal" that I'm being harassed with.
There's no need to elaborate further. It's official: my commentbox hecklers are bored latchkey kids.
Etruscan relative pronouns (for real, this time)
If we're going to talk about Etruscan relative pronouns, let's talk about Etruscan relative pronouns. Not Latin ones, Greek ones or Esperanto ones. In Etruscan, there's no question that the relative pronoun is declined for case as are all known pronouns and demonstratives. In the Cippus Perusinus, ipa is certainly in the nominative case (matching corresponding nominatives ita 'that' and ica 'this') but the question is whether this pronoun's declined according to its role in the relative clause or its antecedent, tezan, which I give the value of 'cippus':
Sleleθ caru tezan fuśleri tesnś teiś Raśneś ipa ama hen.I admit that this is tricky to assert based only on this (despite the fact that my translation is still grammatically valid and contextually sound) but if one is so certain that Etruscan relative pronouns somehow must behave like Latin ones, then I defy such narrow-minded armchair linguists to explain the following on Laris Pulena's sarcophagus (TLE 131):
Χim culsl leprnal pśl varχti cerine pul alumnaθ pul hermu huzrna-treThings aren't so simple. First off, we may wager that pśl is an unstressed type of pronoun because it's spelled without vowels just as postclitic demonstratives are (eg. cl 'of this', tś 'to that'). Second, this pronoun appears to be doubly marked which is normal for Etruscan and observed many times elsewhere. In fact, in a language like Etruscan proven to operate under Suffixaufnahme, in what way can we meaningfully avoid interpreting this sentence as a genitive case agreement between three consecutive elements that I've boldfaced in the above phrase (ie. culs-l leprna-l pś-l)? And if we can see this, then we can see that the role of the antecedent in Etruscan relative pronouns might actually be important.
For now, I've never ever seen a decent and complete translation of this artifact so we're in uncharted territory. However, here are some hints I can most securely offer: χi-m 'and next' and culsl 'of the gate'. Have fun pondering on that. As always, everyone is free to have opinions but may yours be only productive ones.
 Croy, A primer of Biblical Greek (1999), p.164 (see link) gives the following example:
ἀκούομεν τῶν λόγων ὤν ἡ θυγάτηρ σου λέγει Ryding, A reference grammar of Modern Standard Arabic (2005), p.323 (see link):
"We hear the words (GEN.PL.) which (GEN.PL.) your daughter speaks."
li-l-zawj-ayni llað-ayni ya-ntazˤir-āni ħadaθ-an saʕīd-an
"for the couple (OBL.DL.) who (OBL.DL.) are awaiting a happy event"