No-one sets a price for first class tickets between Edinburgh and Kyle, so the exact cost may depend on who you see on the day..
Indeed.
For the outward leg, the cost of a 1st class single from Edinburgh-Kyle is £65.70, if they were feeling generous, they might charge the difference between that and a walk-up standard single, ie £23.80. Some Train Companies would have a policy of offering this but I do not know what Scotrail's policy is.
A booking clerk may offer to sell this, but they may not. See the Guard on the train, as a Guard is less likely to want to refuse your money and may be more likely to show discretion if 1st is lightly loaded and/or if Standard is heavily loaded.
For the return leg, that's simply a case of weekend first as it's a Saturday.