So I was Wondering, Why does I-75 Cross the Miami River in Dayton 5 time when either it could stay in downtown side, or the Western Side of The Miami River, Could anyone Help me out with this?
Also Is US 35 in Dayton up to Interstate Highway Standards?
Ummm... "Geography".
https://www.google.com/maps/@39.7618725,-84.2172097,13z.
Probably better to consider I-75 keeps a reasonably straight course, and the Great Miami snakes back and forth a few times. If I-75 had been constructed to follow the river, there would be a lot more curves than those at the SR 4 interchange (wishing there were English words to translate the German freeway interchange terms "Dreieck" and "Kreuz"!!) By the route chosen by ODOH in the 1950's the city of Dayton wasn't bi-sected by a freeway like some cities. Most of the areas by the Great Miami were older manufacturing/industrial operations. If you investigate using Google Maps' Satellite view, and look around the Stanley Avenue interchange, you'll see the ghost of former ramps inside of which is a manufacturing building... my dad worked there from 1959-1973, and the fun thing about visiting him at the plant was it was in the middle of the former SB 75 exit to Stanley Ave Eastbound... always fun.
.
As for your US 35 question... basically as answered, between I-75 and I-675 it would meet standards... As you head east to Xenia, there are a grade level intersections which could be upgraded to create freeway grade all the way to Chillicothe and US 23. One crazy idea if I-73 ever gets done in Virginia, etc. would be for Ohio instead of running it through Columbus, run it up US 23 from I-64 (with it going 64 to 77 south of there) and then over on US 35 to Dayton and ending. Dayton to Myrtle Beach sounds like a plan.
.
(Goes without saying the US 35 west of I-75 is not freeway for long... plenty of at-grades -- it's a boulevard to its end at Salem Avenue.)