Yes. Some alliance members look fine on paper but have almost no inventory in the fare classes RTW tickets require. Air India is a Star Alliance member with essentially no RTW fare class availability, and South African Airways is much the same. We route around them automatically.
Yes, and this catches people out because the flights are clearly there when you search. The issue is not whether the airline flies the route. It is whether it releases seats in the fare classes a round-the-world ticket requires.
Air India is a Star Alliance member. The flights exist. RTW fare class inventory is essentially nonexistent. South African Airways has much the same problem. We know this from thousands of bookings and route around these carriers automatically rather than quoting you something that will not ticket.
The same logic applies to specific routes rather than whole airlines. Qatar Airways from Doha direct to Los Angeles books at roughly 10%. Split it into Doha to Hong Kong on Qatar (78%) and Hong Kong to Los Angeles on Cathay Pacific (68%), and the trip actually tickets. It is an extra stop, and it is the difference between a confirmed itinerary and one that collapses at pricing.