You can't play more than one sound slot at a time when you have the file open in the Lokprogrammer software. But bear in mind the volume levels for each sound slot are independently adjustable.
"Homebrewing" your own project is very easy. I've just finished making an 201 sound filed with FT, just have to find the time to load it to the decoder.
Regarding Legomanbiffo's 141 file; I emailed DCKits sometime last year when I saw he had the project advertised on the website. To be honest the reply was very vague, all they would tell me was that they hadn't recorded any new sounds for it which led me to believe he was just using the sounds from the ESU website.
The cab door thing sounds like something he has created himself, he's really good at creating his own files, but it sounds a little gimmicky and as you say, impractical!
If you find the Loksound too responsive then try higher values for acceleration and deceleration. Many people don't realise that Loksound use different values for CV's 3 and 4 which equate to about 25% of most other decoders. So if you programme a value of 100 in to CV3 it will be equivalent to 25 on say a Zimo decoder. I usually set my Loksounds at 200 for both CV3 and CV4.