I think I’ve seen most places where leaks can get in on a Classic. Where it comes out is not necessarily where it comes out!
From my experience of dealing with these issues I’m fairly convinced most of your problems are related to sikaflex breaking down.
Number 1 – Forward Cabin. I’ve not exprenced this – yet. Could be broken-down sealant in the screw holes around the forward hatch. Might be worth rebedding the screws with sikaflex. (I’ve needed to do this to stop a leak in the saloon hatch).
Number 2 – Aft Cabin shelf. I’m not sure where you mean. There are two possibilities. I’ve experienced both.
1. The lowest point of the deck inner deck moulding is in the aft cabin. Any water that collects between the inner moulding will eventually make its way there and either overflow the edge, or (in my case) exit past the reading lamp which is fixed there.
2. A leaking porthole will leak into the shelf over the cupboard. The porthole frame needs to be removed and rebedded in sikaflex. Whilst you are at it do all of the portholes, cos they will all start leaking as soon after the first one does.
Number 3 – This has been subject of some debate on the forum. I think it is caused by the main hatch drains being overwhelmed in heavy rain and it gets between the inner and outer deck moulding. I’ve only experienced this once when the sprayhood is off in the winter, but it did wreck my oven. Nowadays I tie a waterproof canvas cover over the main hatch during the winter months (see photo). this has stopped it happening again – so far.
Another possbility is leaking through the large saloon window. (See my article in DN121 winter 2021 page 13)
Number 4 – Under the cockpit. I have had this. Remove the access panel in the aft cabin. Then remove the two bolts on each stanchion post (four bolts in total) for the binacle from the outside. If you can lift the lozenge shaped parts, all well and good and reinsert using fresh sikaflex. Make sure there is plenty of sikaflex undeneath around the bolts underneath. Needs two people, one outside to hold the screw driver, and one below to tighten the nuts.
RE: Teak decks. Original Teak decks are not screwed to the deck, there are fixings used. They can flex if they are not bedded down properly. (See my article in DN no. 116 Spring 2020 page 13.)
I hope this helps.