My feeling is that nothing comes without strings. I assume the stat is so low because another stat got pumped to over 18. Strength 7 comes with consequences, one of which is the inability to carry pretty much anything. I say track it like a hawk, and play it to the hilt. If the notion of encumbrance tracking bores you, though, then you are free to ignore it. There's no rule saying you have to use encumbrance in Organized Play. The player may have trouble with her next GM, though, who would be perfectly within his rights to demand correct tracking.
Regarding NPCs, you most certainly can ad-lib and breathe life into them! It's encouraged! They can have totally different personalities than what is written, if you want. Here's what you can't change: treasure, gold, Fame and Prestige beyond 2 each, and granted boons, i.e. pretty much everything on the Chronicle sheet. PCs may not find every goodie, and that's fine, but you (as the GM) are not allowed to add any additional treasure or bennies other than what is written.
It's also a good idea to not introduce too many side quests other than what is published. Speed is paramount. Remember that these are intended to be played in 5 hours at a tabletop.
Other than that, go nuts!