[susan53]

No - there's nothing "missing" in these sentences. I can see why you interpret the first as ellipsis of "going to" but it's not. Be plus the infinitive is a structure in its own right.

Evidence of this is that be going to can be used in any form, including the infinitive. It's rare but grammatically possible - Eg. I don't think I can be there on Wednesday. We may be going to have an exam that day

However, Be can't be used in the infinitive when it is followed immediately by to. You can't say : *We may be to have an exam that day.

So Be (in finite form) followed by the infinitive is a structure in its own right, expressing the meanings of :
a) arrangements : We're to meet John in the library. / We were to go to Spain tomorrow, but they cancelled the meeting.
b) obligation : You're to get back by midnight.
c) a condition : If we are to believe what she says, it will all be over by Christmas.

The final example that you gìve - be to blame for something is a bit different I think. The meaning is be the person (or thing) that should be blamed for.... The only other verb I can think of offhand that could be used in the same structure would be thank : He's to thank for the problem. There may be others (?) but it doesn't seem very generative. Perhaps these are best treated as idioms.