Koikauttie45rn
Short answer:
Your most convenient connections are IR13 and IR75, as there’s no need to change trains in Zurich HB (main station) with those. Seat reservations are generally not possible on IR (and RE, R, S) trains, only on IC/EC.
And no, reserving seats on a IC5 would not be a brilliant idea, since you’d need to change to a IR70 in Zurich HB after a couple of minutes.