VPN

For both scenario's its still not 100%. Unless you have full control of the ICS instance you will NEVER be absolutely sure how the traffic is generated and moving.
so
In both your scenario its 98% likely that the other computers can access the company network.

Its a security risk.. And as far as I'm concerned far far far too big a risk to allow. If you use a VPN client to a secure network... DON"T use ICS period.

Scenario 1 : A remote user connected to a linksys router (with built in modem)...3 other users also connected to the same router. all the pc's get connected to the internet through the broadband router...user 1 also connects to his company's network through a vpn connection through the same router...the company network admn had installed a vpn client on his system. Now while installing the vpn client on his system the admn forgot to disable internet connection sharing. Now my question to you is that will the other 3 users also be able to get on to the company's network ?

The other 3 users cannot connect to the company's network as those pc's do not have a VPN Client installed on them, moreover these 3 pc's are connecting through a router and the router is not connected through a VPN, hence these 3 pc's cannot access the company's network even though the administrator has forgot to disable the ICS

[Scenario 1 : A remote user connected to a linksys router (with built in modem)...3 other users also connected to the same router. all the pc's get connected to the internet through the broadband router...user 1 also connects to his company's network through a vpn connection through the same router...the company network admn had installed a vpn client on his system. Now while installing the vpn client on his system the admn forgot to disable internet connection sharing. Now my question to you is that will the other 3 users also be able to get on to the company's network ?
The other 3 users cannot connect to the company's network as those pc's do not have a VPN Client installed on them, moreover these 3 pc's are connecting through a router and the router is not connected through a VPN, hence these 3 pc's cannot access the company's network even though the administrator has forgot to disable the ICS