Q.compost addition to lawn
Debating about use of Soilbuilder or mushroom compost to mix in with existing soil. Want to seed again after I tilled and removed an old lawn. But worried about two things: Salinity content of the mushroom compost mentioned here and the nitrogen content of either option. I plan on using centipede grass seed to start my lawn and understand its sensitive to high nitrogen fertilizers?

I would start with a soil test. This will help you determine the best amendments if any needed to get your soil ready for the new grass.
These articles have more information.
https://www.gardeningknowhow.com/special/organic/the-importance-of-soil-for-an-organic-garden.htm
https://www.gardeningknowhow.com/lawn-care/specific/centipede-grass/centipede-grass-care-planting.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/mushroom-compost.htm