If you've spent significant time away from your wife/significant other, you understand the "urgency", although, admittedly, on top of your son's casket after his visiting is still weird. However, add on top of that Jaime as a POW in the Stark's camp and what he went through at Harrenhal, and Cersei thinking she'd never see him again, the urgency is probably even greater (keep in mind that in the books Cersei had her "monthly visitor" at the time which is even more disgusting). I guess what I'm saying is I understand the urgency, even if the circumstances are, for a lack of better terms, "odd".
Keep in mind the Unsullied are being caught off guard by the Sons of the Harpy, either by attacking when they're not expecting it (like earlier in the season when the Unsullied was slain in the whorehouse) or by attacking with such a great numbers advantage that the only play is to take as many with you before they get you (like what happened earlier in the season when Barristan Selmy was killed). I don't know. I guess I see what you're saying as akin to a Navy SEAL getting murdered during a breaking and entering of his home and then someone saying "He must not be that great of a SEAL because he got killed in a home invasion."
The thing is, he didn't sacrifice Shireen for the Iron Throne, he sacrificed her for Winterfell. I get it, at this point Stannis has no hope of capturing the Iron Throne without the backing of the North, and with as much as many in the North hate the Boltons for how they betrayed the Starks, slaying the Boltons and taking Winterfell probably aligns much of the North with Stannis, and after Ramsay's raiding party things are desperate in Stannis' camp. But even if he slays and flays the Boltons, captures Winterfell, and gets all of the North behind him, there are no guarantees.
He still has to get through the Twins (which nobody has ever been able to do while the Freys hold that ground) and the rest of the Riverlands (which, in theory, are only aligned with the North now through the alliance of the Freys and Boltons where previously the alliance was much stronger through the Ned-Catelyn marriage - and interestingly enough the Tullys may still control Riverrun as Jaime Lannister is in Dorne instead of evicting the Tullys from Riverrun. Although technically Harrenhal has been decreed to be the seat of the Riverlands, which means it's actually Petyr Baelish that controls the Riverlands, and since Petyr's family originates from Braavos and it has been theorized that Petyr has familial connections with the Iron Bank, and the Iron Bank has already thrown their support behind Stannis, perhaps a Baelish-controlled Riverlands would support Stannis).
Wow, my mind is going in far too many directions at one time.
Bottom line is I'd have great difficulties accepting Stannis sacrificing Shireen to claim the Iron Throne - but that's not what he did. He sacrificed her to take Winterfell. That really gets to me.
That father you describe is a Father of the Year candidate compared to Stannis.
Does that mean there's still hope for Frey Pie?