Ok, i've had the same problem, when i first bought my IS back in May i noticed after a week or so the front fogs weren't working, light up on dash, fuse ok etc etc
Anyway i noticed water was gettin into the fogs (don't know if you have this problem too?) but to get to the fogs there are two ways, if you go from the front there are two 10mm bolts that you need to undo, still not very easy and be prepared for some grazed knuckles, then the whole unit wil pop out and you can access the rear of the bulb.
The other is as you say take away the lower part of the inner arch, there are bolts but you can just pull out a part of it enough to get to the bulb, just separate the bumper from the arch by pulling it out gently.
When i finally got to the bulbs i found that the bulb had corroded in the lamp and the connector had rotted away from the bulb so i have now bought new lamps but have yet to put them on, just waiting for my knuckles to heel!
Dunno if this has helped but it is possible to do but as you say not as easy as the manual suggests!!
Good luck