I think you're mixing up several different issues and worrying too much (not criticism as we all do this at times!) Try and tackle each thing independently and it will seem easier.
Re guns and grown up games - I do think this is just symptomatic of the neighbour having an older brother rather than sister. There is nothing negative and plenty positive about your ds not doing this! You are doing great if you are letting him play with what he wants to!
With nappies for pooing - ds is 3.0 and when we first trained him he wanted a nappy for pooing in. We used the old choc button and sticker trick to encourage him to use the loo but the thing that worked most was once we actually got him on there we made a whole ridiculous thing of the plop sound 'is it coming is it coming ,wow plop you did it'. Utterly silly but he thought it was funny and still says it himself . Worth making a fool of myself over!
Re buggies - I am a bit of an outlier with this as ds stopped using his at 20 months. It was bloody hard work getting him to walk as he was initially a bit unwilling but within a couple of months of building his stamina up he was walking a good couple of miles at a time. But he isn't the running off type (too clingy) so that is maybe easier for us.
I'm sure others will have better ideas on the running off thing.
Can you get him involved in the shopping? I get ds to help 'choose' things, spot familiar products etc. He's not Gok Wan yet but it definitely helps. Self scan zappers in Waitrose are fab for this as they love scanning the barcodes!
I'm going to go out on a limb here - I don't think it's necessary for kids this age to be getting food spooned into them (with the exception of if they are really quite ill). That is babying a little bit imho. Ditto non spill cups unless out. Start with just an inch at the bottom of a cup and then work upwards?
That said, I guess this doesn't do him any harm in the grand scheme of things and there are worse things! I would be working towards making the switch to open cups/ no more feeding though.
(Ds is not good at putting clothes on himself yet so it's really useful to see what others do at this age....note to self- must get on with helping him do it in preparation for school!)