Bags on seats - there's a simple solution to that dilemma. Sit in the aisle seat. As the train fills up, everyone will avoid you until the last few seats are being taken, by which point if you still don't let someone sit next to you, there's something desperately wrong with you.
I commit a few of these crimes, especially the one about piling aboard a train - this is usually when I'm on a Tube/Rail Challenge and I've reached the platform and there's a train waiting to depart. Also latching onto a particular door - I'll try and guess it, yes, but if I'm wrong I'll try and reach it as the train comes in.
I can also admit to arriving at Swansea on a HST from London with my mother for a connection on to Whitland. We'd previously taken through trains to Haverfordwest, which were 175s and in that respect similar to the units down in London, but this time it was a 153 bound for Pembroke Dock, and they're, as you know, single car. This particular one had to fit nigh on half the people off the FGW HST, and was packed full; I remember wondering if extra carriages were going to be brought from the sidings!
To this day, we refer to it (a single car 153) as the 'tin can', mostly in affection.