I am very surprised at you suggesting that case is the sole basis for the assertion that "There is precedent that suggests that both Revenue and the landlord are powerless to take action against a tenant who "forgets" to remit to Revenue the tax deducted on rents paid by them to the landlord" - I'd go so far as to say it is (a load of) hyperbolics.
For starters an ombudsman's report is no precedent, since as you no doubt know, the Ombudsman simply makes recommendations, and the public body is under no legal obligation to accept their recommendations, (although many tend to do so in most cases).
Secondly, the fact pattern in that case and what you're talking about are night and day; the source of the inequity, as the ombudsman saw it in that case, arose from the fact that the tenant had paid the rent over gross to the landlord due to a lack of awareness of the requirement to deduct & remit. Revenue were sticking her for the remittance notwithstanding that she hadn't deducted.
A tenant who deducts an amount (prima facie evidence of their awareness of their obligation) and then fails or "forgets" to remit, hasn't a leg to stand on, and can take no comfort whatsoever from a 26 year old ombudsman case, as I'm sure you must know.
Unless there's a loophole in the drafting of section 1041 TCA 1997 as amended, that you know about and nobody else does, it seems quite clear that Revenue absolutely do have the power to pursue tenants for tax deducted and not remitted, under section 238 TCA 1997.