You're thinking of futures margin. Futures p/l is handled as a cash flow, not FOPs. FOPs are m to m but as unrealized p/l. FOPs use SPAN or a risk scenario based margin, whatever, with a "one day risk horizon." These differences sometimes cause the margin for FOPs to be greater than the margin for the UL.
IB also can and sometimes does mandate margins greater than the exchange rates.
OP, auto liquidation is a scary reality at IB. I don't like it, it would be nice if IB could find a better way. But it's part of the price you pay to do business there.
The spread has two different ULs so the long option does not cover the short. It's not a calendar spread per se, you're trading the futures spread as well.
The contango in 2008/2009 reached ridiculous proportions. Your spread could lose far more than the debit.
I found this page enlightening.
http://ibkb.interactivebrokers.com/node/990