True enough, but the engine isn't what we're talking about, we're talking about the interaction, the engine tells the NPCs how to act, how to display the environment, what tools they're given to develop the rest of the game, as well as model hookups etc.
The engine hooks up to every part of the game, however it does not influence things like PC action-reactions, dialog trees, NPC counts, etc, an engine is a framework that the gaming company builds around.
Besides, memory management problems, what in the blue blazes do you think a save file is?
It's a database file that the game reads and it stores everything about your character.
As for sloppy coding being unavoidable, it is avoidable, if everyone stuck to doing their documentation on top of the coding, but being a programmer yourself, you should know how often documentation is done.
However, blaming the engine for a design company's laziness, is pure and utter bullocks, the engine (once again) is a framework, if it's C++ Based, and you need something specific that the engine can't do, you make a MODULE, or a FUNCTION, you should know this.
As for Fiscal Applications, there is heads and tails more calculations done on a corp grade bookkeeping program for balancing purposes, not to mention how many decimal places they calculate to, it's effing insane and gives me headaches when I look at anything to do with accounting, and I love logic puzzles. Something about reverse math makes my head all spinny...