But to answer your questions anyway...

1. As to which program to use for the remainder of this year and the next, it just depends on your needs and priorities. As far as I know, the only two programs which do not work on a calendar year basis are LH's Miles&More and NZ's Air Points. But seeing as you seem to be an economy flyer (nothing wrong with that, of course, so am I ) and that you will only be doing 24,000 more miles in 2001, I would suggest you stick to TG for now until the next year.
Of course, if you could stretch it to over 35,000 miles before end-December then AC will give you Elite (=Star Gold) for 2002.

2. Yes, you can definately receive the benefits of Gold whilst accumulating on another carrier. Simplest thing to do would be quoting your TG number until you check in, at which point you make sure they change the number to the new FFP. That way you'll receive the priority waitlist until you change the number at checkin.
Another thing you could do about the priority waitlist, (all other benefits you only need to show them your card, not necessarily put the number into the reservation) is that you can phone up a reservations agent of the airline, tell her that you're a Star Gold member but wish to accumulate on another airline. Ask her to verify (whichever method she will use) that you're Star Gold then change the waitlist status code to whichever they usually have for Star Golds but keeping your new FFP number there and simply making a note on your reservation, "this passenger is Star Gold, number XXXXXXXXX" (phew! I hope you understood all that, it's very confusing I know)

3. Likewise, you can use your TG upgrade voucher by putting your TG number into the reservation until checkin, but you can ask the agent to change the FFP number after you receive your upgraded boarding pass.

You have to make sure that your boarding pass displays the FFP number of the airline you actually want to accumulate on, because should the computers act dumb and drop your miles, you will not be able to prove that you didn't receive credit into the TG account if the boarding pass says "TG*FFP" when actually you wanted to credit the miles to, say, AC.

Hope this answers your questions, happy flying!