Hi Frailty and welcome!
1. Yes, those tubes are fine (though can't vouch for taste/quality) but I heard that the ones that come in small/large tins are/taste better. If you opt for this, then it becomes a lot easier to water it down. Just empty the paste from tin into a container (preferrably an air tight one) and fill the tin with water and empty it in your container.
2. I've used to use taz as my favorite and I noticed that the gravy would be thick by itself, but in a way that wouldn't reduce to a thicker gravy, but instead start to burn/caramelize. I think the reason for those results was adequate watering and blending. Some advice to improve it: blend it twice, water it 1/2 litre further and sieve it before using.
3. I'm myself wondering the same. As I currently do most of my shopping in cash&carry surfaces, which is where all local restaurants seem to source their ingredients, and I don't find the veggies to be any fresher than, say, LIDL. In fact, I find it's the other way around.
In terms of spices, I think you meant TRS. I've never heard of indus personally, but TRS is fairly known and I've seen the viceroy brasserie use some of TRS products.
4. Try
this one. Just be sure to cook the rawness out of those onions. What I generally do is, I use a pressure cooker and have it on the medium flame until it starts to pressure, as soon as the whistle comes up, I lower it to the lowest flame and when the valve goes off, I open the PC and stir things around and then keep the lid half-open (i.e just on top, but not locked) -- if you open early, you may get an explosion of oil/water/veggies -- very dangerous and messy. I had this happen, although only a minor "explosion", sufficient to water the cooker top entirely.