1. The reservations are by campsite. So, does it matter if I have one or four people, as far as getting an itinerary? I'm not sure who is going to be going with me.
Doesn't matter. More than 4 would require 2 sites.
2. The pictures I've seen seem like the spots will generally accomodate 2 2 person tents. Would we have to double up if we had three or four people?
2 tents is optimal. More can sometimes be squeezed in depending. Not sure if there is a maximum # of tents.
3. What do you do the night before your hike? Like in Yosemite, they have backpackers camp areas that you can stay in the night before and the night after your hike. If you're hiking out of Kintla or Bowman, do you have to just start your hike when you can get up there, or is there somewhere to stay?
Depends a lot on time of year, weather, etc. Kintla and Bowman both have campsites but you will compete with car campers to get them. If you can get to camp reasonably early the day before you would have a good chance at Bowman, a lesser chance at Kintla (it is small, only a dozen sites or so). The night you leave the trail will be harder because you will get there later in the day. Look at the campground fill times on nps.gov/glac and make your decision.
4. I've seen people say to get any permit, then try to change it on walk-up. What is the advantage to that? You just have to wait in the walkup line whether you have an existing permit or not, right?
Its a matter of increasing the probability you get the trip you want. Most people want a reservation as close to their dream trip as they can get. But if you ask for your dream trip with no date, campsite or direction flexibility there is a good chance you will end up with nothing. If you are flexible you are more likely to get a good trip but not necessarily your dream trip. So you can perhaps tweak it when you arrive. If you just show up the morning before your chances of getting anything close to what you want are much less. It just depends on what you want.
5. if I decide to get in line at some ungodly hour (4 AM or something) and am first in line, there is still no guarantee I get the sites I want, because people on longer trips that have already started may have grabbed those sites. Is that correct?
Correct, but each night the nps will post a list of sites available in the morning. So you need to get that list and plan your trip based on what is available. If you see something you want in a popular area then get up at 4. If you don't see what you want sleep in and do something else.
6. Is there much of an opportunity to see off trail waterfalls or lakes? I know you have to stay at established campsites, but can you wander on your way there? Or does the terrain dictate how you get there (assuming no mountaineering, etc)?
There are lots of options for off trail exploration if you have the skills. With some exceptions there is nothing stopping you from exploring where you want. But terrain certainly does dictate how easy or difficult these options are. You will see many threads here about various off trail adventures.
7. As far as the backcountry goes, is there a difference in the amount of people you run into between August and September? I would guess that assuming the campsites are all full, once you get out of day hiking range, it would be constant?
The number of people falls off quite rapidly in September. Between late August and immediately after Labor Day not so much difference. Then the numbers decrease fairly rapidly. Also depends a lot on weather and fires, etc.
8. This is probably a dumb question to ask, but are there surrounding areas that are comparable to Glacier for backpacking that are less crowded, or less of a production? I don't mind the people or the paperwork, but sometimes there are places outside park boundaries that are almost equally beautiful.
The Bob Marshall complex south of Glacier is larger than Glacier, has a small fraction of the hikers Glacier gets, requires no permits generally and is a very wild place. Probably not quite the scenic grandeur of Glacier but enough trips to last a lifetime.