I'm going a different direction on this, for short rides, say under 20km or so (12½ miles), your fine in your civvies, more then 20km you need your bike shorts, and nothing under, although you can always wear something over. As for using the creams and butters, the key is early hot spot detection, and near immediate treatment. If your 80km into an 82km ride, your probably okay to finish, if your 80km into a 160km ride, you need to find somewhere private for treatment in the next couple of kilometres. You can use the creams and butters, although something like Penaten cream (diaper rash cream) would probably work just as well. If you find on longer rides, your always getting a hot spot, then I would look at the construction of you bike shorts, maybe try a different type, size or thickness pad.

As for multi-day rides, if you were going on a multi-day car trip, would you take one pair of underwear? Carry a few pair, shorts and jerseys are not big to carry items, the difference packing between one pair and 6, is going to be under a kilo. If your trip is more then a week, plan a Sabbath, an off day every six, where you stay in a motel, have a nice shower, toss all your clothing into a laundry, do any required maintenance, and repack your gear.