Starching fabric makes it go stiff, supposedly to hold creases better. In the age of modern steam irons I find it completely unnecessary and after a couple of cycles of washing and ironing the creases hold themselves anyway.
I’m not sure what you are doing to get shiny patches on shirts. I iron mine before every wear and there’s no shine. Best I can think of is to try turning your iron down (to the three dots setting) and making sure it has water in it. [Edit to add: and the steam setting is on.]
I wear rank slides straight out of the packet and they look flat. The trick it removing them before washing the garment and putting them straight onto another one (so the pairs are kept together).
That’s where I was going with the rank slides. When I talk about transferring them to another garment, that’s invariably an ironed shirt hung up in my wardrobe and not something crumpled up on the floor.
Generally no, but it is possible to stain garments with the spray-on stuff if you don’t use it correctly.
Differently to Flight above, I only use an iron on the absolute minimum heat setting to achieve steam, both for trousers and shirts. Steam the hell out of it, then turn the steam off to go over just enough to dry the garment out again.
I suspect that too much dry ironing is what is causing your shiny patches.
Throw them under a flat cloth with the rank facing down, iron dry for a short stints at a time (they don’t do well with extreme heat or moisture)
Not ideal. Will suit you fine for general wear, but if you’re looking to get an impressive finish then issued DMS will serve you better.