Working jibs are usually in the 90 - 100% LP range, but it's not fixed in stone as far as I know...
You're correct in that a 125% 'jib' is technically a genoa, but the term genoa is often inferred to mean the standard 155% or 'largest headsail' you may have (at least prior to the advent of the newer 'code' sails)
Why 155%? mostly because that's the upper limit for a standard race rating before being penalized (might actually be 153 - depends on jurisdiction and rating system)
A 'class' jib would be one that's legal for a particular one-design class - many of which eschewed genoas, trying keep the class competitive while keeping costs down.
You are dealing with a wide range of different definitions here. A 100% jib fills the entire foretriangle. It may or may not be the "class jib". The class decides what the "class jib" is. It may be a 90% jib, filling 90% of the foretriangle, or it may be a 110% jib, more than filling the foretriangle. A class could have a 125% "class jib". It depends on the class. A "working jib" is another thing entirely. It would be the smallest jib that is used regularly. It would probably be a 100% jib or less; it depends on the boat and what the skipper considers his standard-use sail. Essentially, you have brought up three different ways to descriibe headsails: percentage of foretriangle, class rules, and working sails. They all overlap, so it is confusing. Seaparate them in your mind, and it should become clearer.
A 100% jib doesn't fill 100% of the foretriangle, it just means that the clew of the jib comes back to the mast and no farther (so there is no overlap). It is common for 100% jibs to not have a full luff length and rare for them to come back to the mast along the full leech.
All of these names are just handy names and are essentially meaningless. What really matters is the size of the sail. My "working jib" is 105% and roughly 200sqft. It doesn't have a full hoist luff, the luff is 32' when up to 36 or 37' will fit on my furler. Racers would call this a #3. I could also order a "blade jib" that would be 95% and 200sqft, it would just have a longer luff and a shorter foot.
My genoa is 135% and roughly 280sqft. In racing terms this would be a #2. It does have a full length luff. 135% is a compromise size often used on furling sails, it gets closer to the performance of a 155% #1 but furls down to a 100% "working jib" for higher winds.
A #1 would be 155% and roughly 315sqft on my boat.
When evaluating headsails for my boat I compute the overlap (the percentage) as well as the sqft. One number by itself isn't that helpful, having both lets me better understand how the sail will fit.
Sailnet member chip wrote a handy online calculator for figuring out % overlap and sail area from a few basic measurements: Jib Overlap Calculator
As an example my working jib is 32' luff, 28' leech, 14' foot. The J on my Pearson 28-2 is 11.25'. Punching that all into his calculator I get:
The same sail on one of my friend's Yankee 30 would be 95%, because their J is is 12.75'.