When you see (real) glassblowers you are typically seeing a pot of molten glass sitting in a furnace, then gathered onto the blowpipe, then worked, then shoved into the furnace, worked, back to the furnace, worked, and then finally annealed in (that or another, lower-temp furnace) to relieve the internal stresses. Some glassworkers use a torch to re-heat stuff once out of the furnace, but my impression is that small, handheld torches are appropriate only for very small stuff. If your heat source is inadequate, I suspect it will lead to frustration because it will either require you to hold the torch on the work for a heck of a long time, or, by not being able to spread out the heat, create internal stresses which will cause lots of breakage.
There are inexpensive ways to cut off bottles and such without creating as much of an inhalation hazard that a Dremel tool & abrasive will produce. Also, I’d suspect that getting a nice, even cut around a bottle with a handheld tool is not going to be that easy...and if you can do a semi-good job, then you have to grind the lip and there’s your dust again. There are bottle-cutting kits on ebay, these were sold as hobby items and they are out there.
Allegedly, and I have never tried this: If you fill a bottle to the point where you want it cut off with oil and then plunge a red-hot iron rod into the liquid, the bottle will snap off from the temperature shock. Wear gloves, have a facemask on, don’t have your face over the opening!!
http://www.motherearthnews.com/Do-It-Yourself/1973-09-01/Cutting-Bottles-The-Easy-Way.aspx
I worked at Corning once - took my kids to show them the glass tour and got to do some glass blowing.
When I asked if he had any advice he said “dont inhale”
(but he was serious)