Is there any difference between a 72531‐VE2‐020 and a 72531-VH7-000?
72531‐VE2‐020 = upper blade of a 2-blade set called "Quadracut" used on older Honda HRR models serial number is -8670000 or lower
72531-VH7-000 = upper blade of a 2-blade set called "Microcut" used on newer/current HRR models serial number -8670001 and higher
Between a 72511‐VE1‐020 and a 72511-VH7-000?
72511‐VE1‐020 = lower blade of a 2-blade set called "Quadracut" used on older Honda HRR models serial number is -8670000 or lower.
72511-VH7-000 = lower blade of a 2-blade set called "Microcut" used on newer/current HRR models serial number -8670001 and higher
According to some eBay ads they are all interchangeable.
It is okay to use newer, Microcut blades on an older mower originally fitted with Quadracut blades, but be sure you use a matched pair, don't mix 'em.
FYI, Honda also makes a 2-blade kit, which includes both the upper and lower blades, and the kits have their own part number.
08720-VE2-001 = is the Quadracut blade kit, and 08720-VL0-S00 is the MicroCut blade kit.
Is it generally true that the first 5 digits of this format ("general") Honda part number are the only ones that matter?
No, all the digits/characters matter; sometimes the last three will change when the supplier or material changes, but the parts are otherwise identical.