Also, there is always the fact that "if works to PCs, works to NPCs too". How bad are NPCs fumbles when they attack? My group almost always go with the "end of turn" options, so it is a relief when a multiattack monster fumbles on the first attack ;)
Heavily penalized depends of the result of a fumble. I drop my weapon. My turn ends immediately even if I had other attacks. I beheaded myself. There are different degrees of bad outcomes. And the last one was ironic of course. But if you assume that a fumble on an attack is always terribly bad...
How light works was also houseruled to a torch be better than that. Our GM wants exploration be a thing and not a minor inconvenience waived by magic. It was... strange on the start. By now, we growth accustomed and see some merit on the changes. Perhaps not the regular D&D from most tables, but...
This and where in the world of you are. In most extreme case, Japanese read manga, gamers or not. In Brazil, mangas started to become popular in 90s, when I was around 20. So most younger readers by now already read both western comics and mangas.
My DM enforces it... The houserule is you need BOTH hands free like 2e. So, in an all-human PCs game, no caster wants to hold the torch in a dungeon ;)
There was a time when the Garden of Mortals did not know gnomes, until they began to appear. Out of nowhere. Appearing in mid-air on city streets or in the middle of countryside plantations. Disoriented. With no idea of their past life. It soon became clear that they were fey creatures. Somehow...
10 for something easy, but not trivial; 15 for something from moderate to hard; 20 to something really hard. That is it. No complicated math. Ad hoc in most situations.
If it is really easy (DC 5), I almost never call a roll. Just assume it is a success.
EDIT: Correcting bad grammar and adding...
Of course, as always, you mileage may vary, but one of the better parts of 2e for my group were just that we were not epic heroes. We were heroes. Regular (if above average) people. Not super-heroes in a fantasy setting.
I know lots of people will disagree, but I love how, in 2e, the world did not need set "proper level challenges". You could have a party of 10th level characters that slashes against 1/2 HD kobolds (Dragon Mountain, anyone?) or a 3rd level party that meets a beholder in a random encounter. That...
Another question is how the PEOPLE will see the law change. Most medieval cultures were pretty traditionalist and tended to hate changes of tradition. Of course, this does not need to be the case with your people in your campaign, but it is something you as GM must ponder if yes or no.
If the...
One encounter per day. Two at most. But usually just one. No need to short rest to recharge, because no other encounter would occurs before a long. My group like combat, but hates one combat after another.