Nice job on Archie! I have been using Cetol for a number of years now. For me, the base coat is Cetol Natural Teak (about 2-3 layers). Top coat is Cetol clear, renewed every year. Works well. Incidentally, I did have to replace a section due to an unfortunate case of spring-line rub in a busy harbor, and was pleased that it came off easy, without sanding - a relief as my teak is getting mighty thin!
I know there are many varnish purists out there, and there is no doubt to my mind that varnish is a superior finish - but I am a great believer in the 80/20 rule. With Cetol you get 80% of the finish for 20% of the effort. So if you can afford the time - or pay somebody else - to sand and revarnish with multiple coats of varnish at least twice a year, go for it! Otherwise, Cetol works well...
BTW, I do use varnish down below.
Regarding the splash boards, it is traditional to make a ~45degree cut, outboard=down, inboard=up, to ensure and water splashed on the boars drips outwards into the cockpit, instead of inwards to the companionway.
Personally I have two slides; a large, single piece (which I faced with teak veneer) and a perspex 2-piece.