I think we can stay with [sK] for such a rare form. The 進退これ谷まる entry is rare and marginal and really has no impact here.

@@ -16 +16 @@
-<ke_inf>&rK;</ke_inf>
+<ke_inf>&sK;</ke_inf>

On further thought, I'm removing [sK] from 谷まる in favor of [rK].  It is "correct', and this form *does* appear in yourei, even in recent works. You can even find the off appearance in wikipedia.

There is another recently added entry "進退これ谷まる".  If we [sK] 谷 here, it will looks very strange for it to appear there.  And if we go strictly off of statistics, there's an argument to [sK] 谷 out of the きわまる entry itself.

Note: for statistical purposes, only 窮 and 谷 are actually considered analogous by Japanese people, so their relative statistics should probably not consider the more-common 極, which throws off a lot of numbers.

@@ -16 +16 @@
-<ke_inf>&sK;</ke_inf>
+<ke_inf>&rK;</ke_inf>

進退窮まって	1641	67.0%
進退きわまって	453	18.5%
進退極まって	333	13.6%
進退谷まって	21	0.9%


進退窮まる	1305	65.3%
進退きわまる	433	21.7%
進退極まる	224	11.2%
進退谷まる	36	1.8%

no dictionary entries for 窮, and yourei and google news are exclusively 進退窮, so I'm adding 極 as [sK], even though the statistics might otherwise be high enough to display it. 極 is well-represented on twitter though, so definitely worth at least the [sK]. 

sankoku suggests that 谷 and 窮 are "technically" interchangeable, but doesn't have a dedicated entry for this expression.

@@ -10,0 +11,4 @@
+<keb>進退極まる</keb>
+<ke_inf>&sK;</ke_inf>
+</k_ele>
+<k_ele>
@@ -11,0 +16 @@
+<ke_inf>&sK;</ke_inf>

gg5, chujiten, ルミナス, wisdom

Not always figurative.

@@ -19,2 +19,10 @@
-<gloss>to be at a loss</gloss>
-<gloss>to be cornered in a situation</gloss>
+<gloss>to be driven into a corner</gloss>
+<gloss>to be left with nowhere to turn</gloss>
+<gloss>to be in a dilemma</gloss>
+</sense>
+<sense>
+<pos>&exp;</pos>
+<pos>&v5r;</pos>
+<gloss>to be unable to go forward or backward</gloss>
+<gloss>to be stranded</gloss>
+<gloss>to get stuck</gloss>