Sometimes I think not only is it my right as a 1E DM to houserule differently from the core books but that at times, it is my duty.
There are some rules in which spells will spell out certain effects , limitations, results, etc.. that I just have to bring the whole cart to a complete stop and make things right before we go any further.
Invisibility is one of those things. Both the spell and the ring contain what I consider flawed or at the very least, unclear thinking. One of the main things I had to fix was the whole "attack and become visible" nonsense.
First of all, it is my own opinion that an invisibility spell that ends just because you moved is a stupid and pointless spell. The whole point of being invisible is so that one can move without being seen. DUH.
Now, I thought to myself there must be some kind of reason someone would put that into a book. perhaps it just wasn't clearly communicated. Far be it from Gary's writing to be unclear on things.
So, thinking back to descriptions of people becoming invisible in many books I have read and in movies I have seen, one of the more common workings I have seen is that extreme movement results in the invisible person having a "Shimmer" kind of like the heat "waves" you can see radiating from the road or something on very hot days.
If an opponent were really perceptive, they would see the "Shimmer" and recognize that something fishy is going on here. They might recognize it as an effect of an invisibility spell as they have seen it before. Because of that, they would have a better chance to strike at the invisible person, or at least where the invisible person last was seen "shimmering" and have a much reduced chance of actually hitting them.
Once the invisible person slows down enough to not "shimmer" they would be totally invisible again and the only thing to give them away would be the sounds they make. Unless, of course, being inaudible is also part of the spell, then the invisible one is really able to get away with a lot.
So see, with this "interpretation" I am able to find a way to have an invisibility spell that stays invisible but yet affords a spectator with a good eye the chance to "see" what's going on and not think it's an utterly stupid thing to have happen. I can "see" that.
I forgot to mention that the invisibility spell in 1E PHB is written as an Illusory spell, meaning that the spell is basically an illusion.
I take my basis for my version based on old movies like "The Invisible Man" and others in which the invisibility is more like an alteration, like "polymorph" or something.
I agree, as an Illusion, breaking one's concentration enough to attack would end the illusion. That's what I think is so lame by making it an illusory spell.
Just thought I'd mention that.