I am honestly not getting Segui at all. Here's a photo comparison of the three candidates (including Manguel wearing almost the exact same expression) from the general time period. The funny thing is, the man in question looks most like a young Frank Robinson (maybe a mustachioed F-Robby went back in time to pose in a picture with his younger self?). I'm not convinced it's Cabell by any means, but it's tough to compare because of the differences in angles, the lack of clarity and his tongue (ha ha). On the positive side, the face shape is the same, the hair is the same and his curved slope of a nose ending in a point looks to fit, too. But, again, there's not enough information to definitively determine it either way.
I would say it's very likely the player isn't either Cabell or Segui.
cabellmanguelcomp.jpg
Do you have access to any sort of rosters from this team and/or league from that particular season? Or a clearer photo?
Lastly, the final person you're hoping to identify is the guy on the ground, right? Could he possibly just be an older bat boy or something? All of the players are seated or standing in the traditional team photo set up, and one guy is on the ground in front, a la the bat boys. It seems to me he doesn't share the same 'stature' as the other guys and may not be an actual player.
For extra credit, you should try to name that kid looking on from the stands in the background and whoever is hidden behind Mike Strahler!!