If phosphorus trioxide is present in the atmosphere of Venus, could it not react with water to form (sufficient) phosphorous acid? Sufficient phosphorous acid to be decomposed in the measured quantity of phosphine ?
From Phosphine gas in the cloud decks of Venus:

We also rule out the formation of phosphorous acid ($\ce{H3PO3}$). While phosphorous acid can disproportionate to $\ce{PH3}$ on heating, its formation under Venus temperatures and pressures would require quite unrealistic conditions, such as an atmosphere composed almost entirely of hydrogen.

But according to Vega mission results and chemical composition of Venusian clouds there is $\ce{P4O6}$ in the atmosphere which according to Wikipedia would react with water to form phosphorous acid.
However there is supplementary information in the chapter &quot;Equilibrium thermodynamics in the atmosphere and surface&quot; at the end of the article &quot;Phosphine gas in the cloud decks of Venus&quot;:

As an example of our approach, we present a calculation for phosphorous acid ($\ce{H3PO3}$). This compound will spontaneously decompose on heating to form phosphoric acid  and phosphine; this is a standard laboratory method for making phosphine. Phosphorous acid is not stable in gas phase, but could in principle be formed in cloud droplets by reduction of phosphoric acid.

Edit for some additional informatiom.
Also from the supplementary information (page 13):

Reactions of $\ce{P4O6}$, $\ce{P4O10}$, $\ce{H3PO4}$ and $\ce{H3PO3}$ were considered (the last of these only in solution phase in the clouds),..

So not considered was the possible occurence of $\ce{H3PO3}$ below the clouds.
The Recent Evolution of Climate on Venus (page 23) states:

The evaporation of $\ce{H2SO4}$ occurs at about 48 km, the average cloud base.The vapor phase continues to exist down to 432 K (38 km), where it is thermally decomposed.

So below 38 km, and above 160 ⁰ C there would be water in the gas phase, free from sulfuric acid, that could react with $\ce{P4O6}$.
Could have that possibility been ruled out for some reason ?
The issue seems to be not the reaction between $\ce{P4O6}$ and water, but whether $\ce{P(III)}$ can form at all.  The OP's first reference says no whereas the second reference says yes.  Basically the two references used different thermodynamic assumptions.  The first one assumes an atmosphere in equilibrium while the second assumes only a partial equilibrium; in the latter case $\ce{CO}$ is allowed to form from nonequilibrium (radiation-driven) dissociation of $\ce{CO2}$, and minor species are then equilibrated with the $\ce{CO/CO2}$ couple.  In the case of phosphorous oxides this favors $\ce{P4O6}$ over $\ce{P4O_{10}}$, although the former could be oxidized by other species such as sulfuric acid.
The issue has become important because the recent discovery of $\ce{PH3}$ among the clouds of Venus has led to a hypothesis that this compound is generated by Venusian microbes, but if $\ce{P(III)}$ is available for a disproportionation reaction it could provide an alternative abiotic (non-life) source.  As noted above, the references disagree on this, and the question of whether this alternative to a biological source of $\ce{PH3}$ is viable awaits resolution.
Relevant to the question of whether $\ce{PH3}$ is a true biosignature is this question about organic compounds on Venus, which apparently spawned the current one.