There is no alternative functioning government without Maoists support. They knew it so they went for such a dangerous step alone.

About presidental rule: it is impossible to do so by president alone without support from all other political parties. Next, NC will lose its identity as a democratic force if it invites presidental rule with the help of army. also, if NC can obstruct the CA for 2 weeks, some people may argue why Maoists can not do that. if army rule really become infamous, NC would invite the end of NC from Nepali politics. Maoist could take the advantage of situation and do some revolt. Army is sort of divided now, so presidental rule is not possible.

now, I heard that India wants or likes Baburam Bhattarai to lead Maoists or government from long ago. he had good relationship with Indian leaders, especially with communist leaders. if Maoists hard liners did not oppose it, Baburam would be the prime minister instead of Prachanda. Times of India wrote few days ago that India would be engaging on talks with Maoists to please them. India itself will have hard time dealing with the situation if Maoists are out of power in Nepal. So, India might have given some signal to Prachanda about it, so Prachanda might have said that Maoist would be leading next government.