RAID
Get the defective Nest alarm replaced under warranty and put it on fleabay.
Buy a set of detectors from an established manufacture so that each of the bedrooms, living areas and as much of the house where there is a possible risk of a fire starting (e.g. utility room, garage, attics, basements) is covered.
As for CO detectors, again sited according to risk and need to alert occupants.
Don't forget to test them regularly, Once you know the battery life, replace the batteries pre-emptively - then no more waking up in the middle of the night due to low battery warnings.
The total outlay may well be over the cost of a single Nest detector - but you are not reliant on a single point of failure of a complex device. About the only sophistication that would be good to have is the ability to link them. Redundant Array of Inexpensive Detectors
Biting the hand that feeds IT
