I wonder how to count the non-guided ascends, as there is no registration etc. at all for climbing Montblanc.
Technically the normal (Gouter) route is easy, the main challenge being altitude and stamnia. So a guide wouldn't help that much here.
IMHO the normal route is definitly easier than Island Peak (which is quite steep ice nowadays AFAIK) but more serious than Californian or Colorado 4k peaks, as far as the climate is concerned. The weather at Montblanc can change to arctic conditions very fast, even in summer.
Btw., altitude adaption is a funny game... if you spent weeks at altitude like eg. in Nepal you may find it simple. Coming from sea level and immediately tackling Montblanc is a different setup. Of course here the retreat is extremly simple, compaired to the himalayas. Neverthelesse it should be kept in mind.