If GW were to not complete his term, for whatever reason, would Cheney always become pres
(assuming he is alive and unimpeached himself)?
Yes

(can the VP be impeached?)
Yes

If the President and Vice President are both removed from office for any reason, the Speaker of the House of Representatives becomes Acting President. In practice, that is as far as succession can go, because, if the Speaker dies or resigns, there is always a new speaker.

On a related note, with the dissatisfaction among repubs in DC, if the US had a parliamentary system, would a non-confidence vote succeed? ie, would enough Rs vote against GW to make a majority with the Ds?
Not as long as there is a Republican majority in either house.

(In parliamentary systems, as in Britain, Canada, etc, if the Government in power has the majority of members of parliament vote against them, they are required to call an election immediately. This happens most often in a minority gov't situation - as with the last two elections in Canada, and probably the next - but could happen in a majority if enough members were upset with the leader's/cabinet's performance)
USA has no interim elections except upon death or resignation.

On another related note, if enough of their constituents wrote/phone expressing a certain opinion, would the representatives in Washington vote according to the constituents wishes?
Likely, but not certain.

Is there a "recall" mechanism in the US? (in BC, if 40% of the registered voters in an electoral area sign a recall petition, a by-election can be called and if the incumbent loses, they are out).
Each state has its own laws on this. There is no federal recall in the USA. There should be, but there is not at present.