Is what the father said about the intention to speak only about disbelievers a Bid‘ah (innovation)? And is avoiding speaking about Muslims altogether with disbelievers considered a Bid‘ah? And does speaking with disbelievers lead to disbelief?

The questioner is severely afflicted with waswasah (whispers/obsessive thoughts). Her cure lies in completely ignoring these whispers and refraining from asking about them, while earnestly supplicating to Allah. Merely speaking with disbelievers is not disbelief (kufr). Avoiding criticism of some Muslims' actions in the presence of disbelievers has a good rationale and is not an innovation (bid'ah). It contains benefits, including: protecting the reputation of Muslims from being tarnished, and because excessive negative talk about some Muslims, even with other Muslims, contradicts the principle of concealing faults, and it might gladden the disbelievers, and there is a fear of falling into spreading indecency.