The 60xt stays on. The 60a has a tab that makes it much more convenient to take off, but it will also catch on hair or meat and slide off while using it...nothing like searching in a rear quarter for a lost scalpel.
We have built many, as stated you need a thermal break and vent your heat source to the outside. It really is that simple. An RV furnace will work very well, but does require 12v and LP.
I prefer a lid, it is nice to put things in I need quickly or things I don't want bloody on a pack out. It also allows a cleaner and tighter set up, I'm not familiar with a lid flopping around at all.
There are very few extended warranty plans that are worth much in the RV industry, and in my opinion none if purchased for an RV that is over 3 years old. Typically, if it pays 75 cents on the dollar, it is considered a top tier plan. Even these will have a list of exclusions included, but not...
Being in the RV business I am not particularly a fan of extended warranty plans. Having said that, complaining that the warranty doesn't cover general maintenance when it states it doesn't is hardly a reason to be upset. As to your comments about silicone, I really hope they don't have a EPDM roof.