I would say the decorating etc will take as little as 15 mins, tho make sure you leave the stuff out (if practicable) as some will want to return and titivate their creations.
Our last party I had to say to the few stragglers "right OK finish that later, it's time for musical chairs". That was fine tho.
Your games sound fabulous - great original ideas! - but I don't think 3 will be enough. Most games last 10 mins at most esp with 3-4 yos. For DS2's party this year (he was 4) we had craft plus at least 6 games IIRC. I had a spare one up my sleeve we didn' need. Food and cake will take about 20 mins prob.