Before the fifth invasion Enweil + IVF had more economic strength, a higher average combined CS and /far/ better food production. Enweil alone had near our total CS. If IVF had supported Enweil efficiently (as they did state they would) then we'd have been in a lot of trouble.
And on one point I massively disagree: Enweil had the advantage of interior lines. Stationing an army in Enweilios they could strike against Kuugl within 2 halfdays - and against Fwuvoghor in 3-4 halfdays. While Riombara could either station an army at one end of their realm (Kuugl or Fwu), then march 8 halfdays towards the other end. Or station an army in Rines, reaching Fwuvoghor in 4 and Kuugl in 5 halfdays. Or, counted differently: distance between the two borders between Enweil and Riombara: 4 regions for Enweil, 9 for Riombara. I can tell you I was /very/ glad Enweil did never capitalize on this.
I am considering movement rates in this, too. Riombara always had a much more reliable army than Enweil had. The secession was also a serious mistake, as even if it reduced distance from capital penalties, we were stuck with a rogue-plagued realm with a far-off capital, and Enweil was left without proper military leadership.
Also, as for the economy, the estate systems considerably favored compact realms with big cities and townslands like Rio over vast rural realms like Enweil. Our estate efficiency was atrocious. It's true we had more food, but you never seemed to be starving, so that doesn't really change anything.
Yea, Fwuvoghor wasn't to far from Fengen... But it was fortified. When we owned Fwuvoghor, we did launch strikes from there. But even then we had to be careful, because every attack from the north left the south exposed. And Rio could attack from any two regions in the South, it was impossible to truly block you, and the only buffer to Enweilieos was Lopa (which was eventually lost), meaning that a strike on Enweilieos could be followed by a takeover.
You seem to overestimate the advantages of striking through the North... Last time we did it, barely any troops survived and almost all nobles who participated in bypassing Fwuvoghor ended up captured. Sure, it would take you a long time to reach them if you left from Grehk... but that would be a stupid move to begin with. If Enweil attacks you via Fwuvoghor, your best move would have been to strike at Enweil's south, not to waste your time chasing a battered army in the North.
As long as Fwuvoghor was Rio's, there was nothing to capitalize on.
I do find it ironic that it now seems that Rio is running out of food, and may starve itself. After all that happened.