* Houthis seek to pull rivals into deal on power * Two groups walk out, highlighting distrust * Collapse of talks would Risk failed state, militant haven By Mohammed Ghobari and Angus McDowall SANAA/RIYADH, Feb 9 (Reuters) - In talks that could pull Yemen back from the brink or pitch it into full-scale civil war, the powerful Houthi movement will attempt this week to draw rivals into a new presidential council to run the Arab world's poorest country. Failure in the negotiations could spell a surge in violence, a humanitarian disaster and the collapse of Yemen into a failed state from which militant groups like the regional arm of al Qaeda, which has attempted attacks on U.S.-bound planes, can plot and launch operations. Two weeks after President Abd-Rabbu Mansour Hadi and his cabinet quit, leaving ministries directionless and efforts to save the country's shredded finances in limbo, it is increasingly clear that Yemen needs a government.