The 1874 RLDS edition accidentally changed the past-tense supposed to the present-tense suppose. This obvious typo was corrected in the subsequent RLDS edition (1892), probably without reference to any other edition. The rest of the passage is in the past tense (“separated ... departed ... should meet … was ... had undertaken”). For another example where supposed was accidentally replaced by suppose (in this instance, when Oliver Cowdery originally took down Joseph Smith’s dictated text), see under Alma 56:37.
Summary: Maintain in Alma 17:13 the past-tense supposed, the reading of all the textual sources except the 1874 RLDS edition.