Even if it’s not such a great looking Tarkin, I would have loved to see more of him in ROTS. Just a couple of scenes, nothing more.
About Mon Mothma’s age:
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In ROTJ ("+4"), let’s assume she’s 50 yo (Caroline Blakiston, who got the part, was 50 yo in 1983 - she still lives nowadays… caméo in the ST could be a possibility);
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In ROTS (deleted scenes, “-19”), with the assumption she’s 50 in “+4”, she’s 27 in “-19”. Genevieve O’Reilly, who took the part (and who still has it in Rogue One), was 28 yo in 2005;
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In Rogue One (year 0, roughly one or two weeks before ANH I guess), she’s then supposed to be 46 yo. Genevieve O’Reilly is 39 yo this year.
So the age difference is not really an issue. She looks younger in Rogue One but not that younger. Having to lead the rebellion after the death of Bail Organa was no easy task.