You can enroll in a 141 course, and it is usually an instrument and commercial course combined. The catch is that you will have to complete the whole couse, which is 160 hours or so. So if you already have a bunch of hours towards the part 61 minimums of 250 then you are probably better off staying part 61. You can't use any of your previous hours toward the 141 course, it all has to be completed while enrolled in a 141 school, using their 141 airplanes and 141 instructors. However, I did my certificates much the same way several years ago and found the 141 training far exceeded the part 61 I had previously had as far as quality was concerned. One of the best options may be to find a 141 school that also does part 61 training and will give you the same quality of training.
Yes you can, but I believe the rule is you can only use half of your part 61 training/flying. Depending on how greedy the school is you can complete your training by completing several lessons at once. So even though you have to complete all the lessons, your first flight could be lessons 1-5 if you demonstrate enough proficiency to move on to lesson 6. Some schools will even do a "placement" flight to decide where you should start in the syllabus.
So really, the only thing you have to decide is will you have to fly more (i.e. spend more) part 141 or part 61. Here is the kicker. If you have 120 hours it makes absolutely no difference. If you have 120 and you go 141, you can use 60 hours towards 190, leaving 130 hours to be flown. If you go part 61 with 120 hours you need 130 hours to reach 250, so its a draw. Either way you'll end up at 250 hours, so what it really comes down to is who is going to charge you more for the airplane!