So is the old TF KogMaw still viable?

Trinity force became a good item on kog'maw when they changed phage to give movespeed. This allowed kog'maw to kite really well. Now that kog'maw has to stand still to use his w, Kog'maw thinks more about what will give him the best damage and rageblade seems to have taken that spot.

Yes. Sheen is very powerful in winning early trades.

If Riot had gone along with the idea of replacing Zeal with Recurve Bow, then Kog would have loved it.

Sadly, champions who build Trinity don't like auto-attacking a lot according to Riot.