the questions aren't dense they are quite valid.
Other people will have different opinions and experiences but for me when it comes to powders I don't like them unless I want to feel really babyish. they are basically meant to reduce friction from movement and help absorb moisture but I find they go pasty/clumpy/sticky and make it a little harder to clean properly. Also disposable nappies are meant to wick urine and hold it away from skin but I think the powder will absorb it and sit on your skin so that can't be helpful.
As for lotions/oils these should be fine to use every change as they basically just create an extra barrier between the nappy and your skin also removing friction and helping prevent any mess (if you do it) from sticking to you.
With the medicated creams like sudocrem/metanium/bepanthen/balmex (all uk Brands) etc. you should probably only be using it if you have a rash or know one is developing. I have known parents who use sudocrem or bepanthen at every change without any problems but the directions on the tub I have here say apply as required. You only need a thin layer for it to work, no need to slather it on and also make sure the area is definately clean and dry before applying it.
You can use the cream any time, if you have a rash you're better off going without nappies for a bit as it lets the area air out, just add a thin layer of cream before putting a nappy or your normal clothes on. If your not using a nappy make sure to wash yourself and dry well after a little while and reapply as you would with a baby if you were regularly changing their nappy (when I've used sudocrem without a nappy I wash + reapply every 3 hours or so if possible), do this until it's cleared up.
Also to note here is that some nappy rashes can get really bad and may be caused by a fungal or yeast infection, if you do ever get a rash and after a couple of days letting it air and using OTC creams it is not getting any better see your doctor as they will be able to prescribe a treatment. You may never experience this but its good to know in case.
hope that helps.