Malcev completion conjecture for the kernel of the evaluation map

Let $R={\mathbb Z}[t]$ or $R={\mathbb Z}[t,t^{-1}]$, let $K(R)$ be the kernel of the evaluation map $SL_2(R)\to SL_2({\mathbb Z})$, and let ${\mathcal W}$ be the kernel of the induced map from the relative completion ${\mathcal G}(R)$ to ${\mathcal G}({\mathbb Z})$. Malcev completion conjecture. The map

$$K(R)\longrightarrow {\mathcal W}$$

is the Malcev completion. The group ${\mathcal W}$ is prounipotent, providing some evidence for the conjecture, but the claim is presented as conjectural in the case $n=2$.