How much closer would they have been in their 75 year journey, theoretically, before the final shortcut landed them home in one jump in "Endgame"? Without further theoretical shortcuts, where were they relative to where they had started 7 years before?

Basically halfway home. Maps glimpsed onscreen would place them at the Delta/Beta Quadrant border, and the sum total of jumps doesn't exceed that traveled distance, while regular travel nicely fills the remainder.

So, thirty years or so still to go, barring shortcuts.

In "Q Who", when the Enterprise is sent hurtling into the Delta Quadrant, Data estimates if they head back now, it would only have been two years or so

No, System J-25 is not in the Delta Quadrant. We get an explicit reference to the distance: 7,000 ly is not yet sufficient for moving from Alpha to anywhere but the neighboring Beta.

OTOH, covering 7,000 ly in 2 years and 7 months is only doable theoretically, "at maximum warp". We know starships can't maintain maximum warp for long.

Similarly, in "Where No One", 2.7 million lightyears could be covered in mere 300 years "at maximum warp", but the ship simply can't do maximum warp. Not for more than a couple of days at the most extreme.

and of course Guinan's people had been here before (and were close enough to be picked up by the Enterprise-B in Generations).

Yes, Guinan used to live at J-25. But no, she wasn't there when the Borg came - this is why she survives. Nobody knows who picked her up and where, and how long it took for her to reach the immediate vicinity of Earth where the E-B finally took her aboard, and what sorts of legs that journey involved.

So, if they had high-speed floored it back to the Alpha Quadrant and had a good run, they'd have gotten back around the timeframe of TNG season 5.

Except they can't move that fast. The longer the distance, the slower the speed, apparently: they have to factor in an increasing number of pit stops, cool-off periods and the like.

From what we actually hear in the show, a hundred lightyears in a day is a breeze; a thousand lightyears in a week is an ordeal (save for one TOS outlier); and ten thousand lightyears takes more than a decade. It then apparently evens out, so that crossing the 100,000 ly galaxy takes about a century - but a million lightyears doesn't take all that much longer, perhaps because ships travel faster between galaxies than within them.

Timo Saloniemi