Why does this reaction not go to completion?

[PoochGroup]

Is my reasoning correct for this question? The tert-butyl group locks the ring into a single conformation where it is in the equatorial position. SN2 reactions require a backside attack and electronic effect (sterics from the hydrogens) prevent one of the reactions from occurring.

 

[hypervalent_iodine]

Small note about your first question: your equatorial bonds almost look axial. Either draw them a bit more angled out from the ring or draw in the hydrogens to make it a bit clearer.

 

Your second one is also right. Yes, the t-Bu group locks the ring, and reaction requires backside attack of the brominated carbon from the deprotonated OH. In 1, the OH is axial and can easily attack from above this position. In 2, the OH is, as you have correctly drawn, equatorial. It is repelled by the axial hydrogen from top-down attack and it doesn't have the flexibility or rotational freedom to move up to where it needs to be.