PS i found link that explains the greyed out install butt on sideloading apps - it is something amazon broke in fire OS 5.6 but easy to workaround
it applies to all tablet on the latest fire, which explains why I saw it on both the HD 8 and the HD 10
para 1 in this article
https://teleread.org/2017/10/14/how-...ew-fire-hd-10/
"UPDATE: Amazon’s most recent Fire OS update, 5.6.0, has broken the install button for third-party applications. Fortunately, there are a number of simple workarounds..."