Hi there!
I think that you've got a very nice and interesting plan for Japan, let's have a look at ticket prices and see if we can match a JR Pass to your travel.
Tokyo - Toyama ¥ 11,910
The JR Pass covers travel up to Toyama, from there you can use the Toyama Chiho Railway Tateyama Line for the last part to Toyama (¥ 1,200yen).
Toyama - Matsumoto ¥ 6,750
Matsumoto - Nagano ¥ 2,840
Nagano - Tokyo ¥ 8,200
Tokyo - Otsuki ¥ 2,820 covered by the JR Pass. The last part to from Otsuki - Kawaguchiko is travelled on the Fujikyu line which requires a separate fare of ¥ 1,140yen
Osuki - Tokyo ¥ 2,820, return (same as above)
Tokyo - Kamakura return ¥ 1,840
For a total of ¥ 37,180 or ¥ 8,070 in savings as the 7 day JR Pass is ¥ 29,110. In addition you can also use the JR Pass for local travel around Tokyo and travel to the Airport, possibly adding even more to your savings.
Hope this helps!
Daniel