Wow what a run-on sentence! :P
Longer answer: It is within the federal govt's authority to write the labor agreement from scratch. They could have used the workers' preferred agreement.
In fact, the bill the House of Reps passed did include the seven sick days. The Senate held a vote on whether to include the sick days in the deal; it received a majority of the votes but due to Senate rules (filibuster) that was not enough, and thus the provision failed.
The government went with "lower costs for consumers"
Why "mandate on-call employees to cover sick employees" wasn't an option, I don't know.
It doesn't have to raise prices. The costs imposed by the sick days (millions) are absolutely dwarfed by annual profits (billions).
I'm just hypothesizing here, and I feel like I am missing something since it doesn't seem like having/adding on-call employees was an option.
They would have considered everything you mentioned in their cost estimate.
This situation does not lend itself to pat answers, including why Team Pelosi didn't first try a bill with both provisions combined. This trainwreck (sorry) could be seen coming for weeks, there was plenty of time to try to do the right thing. Except of course for both parties' establishments hating this segment of the population.