For #1
It's not that photographers "never upgrade" their camera bodies. It's that you'd want to have good reason to do so. Having good reason means you understand the differences in the various bodies, have a type of shooting you do which demands certain features, and then buy a body based on it's ability to satisfy those needs.
For example... if you're shooting sports photography, you probably want a good focusing system and a fast continuous shooting speed. The 7D, 70D, and 5D III and _really_ the 1D X (the flagship) all satisfy this need to some degree. (Incidentally the 7D is ripe for replacement sometime later this year.)
If you find you're shooting in low light and have no control over lighting (e.g. shooting weddings in poorly lit churches (hint: all churches are, by definition, poorly lit. If you look up "church" in the dictionary, the definition starts with "A poorly lit building in which people go to attend religious ceremonies.") If the building has good lighting I think you have to call it something else. Alas, I digress. In those poorly lit shooting circumstances, you often find the need to crank the ISO and in so doing, a typical camera produces a lot of "noise" in the image. It turns out (for reasons of physics) cameras with physically larger sensors tend to have a lot less noise at high ISO. A Canon wedding photographer might gravitate to the 5D series body, or 6D (on the lower end) or 1D X on the higher end.
Your T3 is the entry level body for the entire Canon DSLR line... so it's features and performance are minimal... other bodies will tend to have better focusing, faster shooting, etc. You will not notice a bump in ISO/noise performance until you get to full-frame bodies (not entirely true... the 70D does offer a bit of improvement but nothing that jumps off the page.)
For #2
Your current "kit" 18-55mm lens will work for big group shots (you don't need (nor want) low focal ratio here... minimum f/8 to f/11 to get the depth of field depending on the group size). But your examples are varied. I'll be honest... the 75-300 and the 50mm f/1.8 (specifically the f/1.8 version) are designed to be very inexpensive. The 75-300 is perhaps the most mediocre lens you can buy in the Canon lineup. The 70-300 USM is much much better. The 70-200mm "L" series lenses are _vastly_ better. The 50mm f/1.4 is significantly better than the f/1.8 version in terms of build quality and the quality of out-of-focus blur. The f/1.8 exhibits a "nervous" or "jittery" bokeh quality due to it's odd shaped 5-blade aperture.
For #3
You should have lighting. Photography is all about lighting and lighting, by definition, also means shadows. To create shadows (especially good looking shadows) the light needs to come off the camera and come from some other direction. It also needs to be a broad source light instead of a pinpoint source to allow the light to gradually transition to shadow. Pinpoint light sources create well-defined edges at the light/shadow transition and that generally does not look good. Some off-camera flashes, a way to trigger them, and a way to soften them (and by "soften" I really mean "broaden the source") will do wonderful things for an image. Lighting can really create moods and drama in the image. You can minimally start with a "shoot through" umbrella (white fabric which is translucent so when you fire into the umbrella, the whole area of the umbrella lights up and that's what illuminates your subject.) A shoot through umbrella is probably $10. Better would be soft boxes because they offer much better control, but they do cost more.
For #4
No opinion here - lots of good labs.
For #5
When someone says they prefer natural light what they really mean is they don't like the edgy harsh look of flash lighting. But you can create stunning (frankly better than natural light) looks if you learn to use flash and control the light... diffuse it (broaden it). If you've got a white and preferably low ceiling, simply pointing the speedlite (must use an external speedlite... not the built-in pop-up flash) at the ceiling will do wonders for the look (because now you get a shower of light coming down from above.) A small bounce card (sometimes called a catch-light card because it also adds a twinkle to the reflection in your subjects' eyes) will help push some light under the eye-sockets and chin to remove the "raccoon eyes" look that you can get if the light comes from directly overhead.
I was having a bit of fun shooting a car show with a buddy. Actually I think my buddy was getting the most fun because I brought a large collapsible 4' diameter reflector. I brought it so I could shine light into the engine compartments and also into the interior for MUCH better illumination of the engines and dashboards, etc. Unfortunately this meant I did all the holding of the reflector and he got the benefit of the shots. You will need an assistant if you use a reflector but it's a great way to improve the lighting. I use them near windows... if I have a bride looking out a window with natural light on her face... it also means that on the side of her face away from the window she'll be pretty dark. A simple collapsible reflector held by an assistant just barely out-of-frame fixes that without the need for flash. It's been a few years, but I'm thinking my reflector (and I have a few of these in various sizes) was about $30 (incidentally... if you only buy one avoid the "gold" color. Those are great for enhancing the look of a suntan when you're shooting models on a beach... but for most other situations it's going to provide an awkward color cast.)
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