. . . I downloaded the manual, but it's futile to follow instructions. . . .
(a) Start by doing a "factory reset", if the manual tells you how to do that. That should eliminate the "split keyboard" problem.
Some DP's have some voices that are _inherently_ split -- bass guitar in the left hand, piano in the right hand.
. . . Are you using one of those voices?
. . . Are you in some kind of "set" or "performance" which specifies a split keyboard?
(b) There's probably no way to start the recording -- using rhythms -- before you press the first key, which starts the rhythm. That seems strange, but I can imagine the arguments on both sides.
So, do what the manual says -- select the rhythm, set the DP to "Record", and start playing.
Maybe I don't understand what you'd like to do . . .
(c) There is no alternative to reading the manual, as many times as it takes to understand it. The Casio manuals aren't too bad, in my experience. If it makes you feel better, I remember scratching my head a bit, when I got the PX-350, figuring out how the "Record" function worked.