Not so simple. See - the Lib Dems made a deal with the Tories 2010-15 - then they were wiped out. Their base felt betrayed. This was caused by ideological differences. They learnt the lesson and will not do it again.
ODM was the biggest party for a long time - 2008-2017. Raila did not become president. This is because coalitions are based on many factors - ideological differences with URP (liberal vs conservative), competing constituency in non-Gema, domineering personas that conflict, etc. Ruto is very likely to continue to be isolated as he is right now. Noone wants to back anyone who then directly takes over their constituency as Ruto has attempted to do in Mt Kenya. Raila attempted this in Kalenjin 2008-12 and it cost him the presidency. The "big groups" of pastoralists (URP) and the Raila coalition of Luos-Luhya-Mijikenda (ODM) have too many points of conflict. But insular Gema don't threaten your base.
Expect Uhuru to get more offers of coalition than Ruto and Raila combined. Gema insularity is a strength not a weakness.
True. Having "fewer" MPs than population is a question of whether representation can be said to be fair (democratic) or not. It doesn't prevent Uhuru being leader of BBI Party or whichever party they form with Raila. However my point is that having fewer MPs behind you then being appointed the leader of a party/coalition negates whole point of a parliamentary system. It's like Labour's Party Leader Corbyn supporting Nicola Sturgeon for PM yet Labour has more MPs.
Second, at the moment if it were a Parliamentary System Ruto would actually unseat Uhuru. He would just withdraw his support and maybe make a deal with ODM and he takes over as PM or we go for elections. That's how dicey parliamentary system is.