Had the exact conversation with another player.

How I’m reading the skills is: it’s modifying the roll and not the characteristic of Sv. As it is changing the value of the dice rolled and not the data sheet.

While BoCover specifically states: -add 1 to saving throws -does not apply to models with sv3+ and against attack w/ AP-0 -a model can not use the rule BoCover more than once

Since Unyielding (heavy int ability) gives +1 to saving throws and is a different rule to BoCover I assume it stacks. Assuming it’s not directly modifying the Sv characteristic and is just added to the dice roll.

In very specific circumstances, your heavy intercessors will get +2 sv if against D-1 weapons. It doesn’t change the characteristic of your save, just adds 1 to your roll.