My sympathies, graraps: it seems that, in Central America, the departure of COPA from SkyTeam will leave a lot of 'holes'. The best option I could find for MEX-CCS (but only a quick search) was to get to GDL, then formally book GDL-(USA)-CCS (with DL) but ignore those sectors; maybe you can get a LCC ticket back to CCS (or CM via PTY).
-- Henry