Waterproof bib tights

Endura Stealth tights are totally waterproof - got the Lite version as the regular ones have fleecy lining more suited to sub-zero. Stiffer fabric means they do feel odd and less suited to high-intensity road cycling but better for commuting or up-to-your-shins-in-filth MTB rides.

I agree about winter boots. So much better than sumer shoes with shoecovers and trying to block up ventilation holes.

They're not perfect, being defeated by downpours running down your legs and soaking into your socks, but I treated myself to a pair of Northwave Artic waterproof boots last winter. They're ace. Far superior to overshoes- no leaks on the bottom, no zips to fail, no struggling to get them on etc. Available in a three-bolt version.

I've got these: http://www.rideworld.co.uk/product/ride-2014-aqua-zero-bibtights/

Keep me dry regardless of the rain. I do the same as you're suggesting - put them on over some Sealskinz. Shoes get wet but I stay dry.

I sealed up my cleat holes with silicone bathroom sealant - as for splashing getting into the tops of overshoes - what about a pair of gaiters?

They don't exist in my experience.
There are a few that have fancy sounding coatings that make water roll off initially, but after a few washes (and after the magazine reviews) the coatings all wear off and you have a normal pair of bibtights.
Happy to be corrected on this, but I've tried a few brands and always found this to be the case.

As above, money is best spent on mudguards, overshoes and warm socks first. When looking for decent winter tights, I suggest to concentrate on warmth (when wet as well as dry) and windproofing, try and get ones with stretchy ankles and a zipper too, so they can be put over the top of overshoes instead of funnelling all the spray into your shoes! That is why I still wear my no-longer-water-resistant fiandre no-rain bibs, the ankle design is good.

If you are dead-set on waterproofing your lower half, look into waterproof trousers rather than tights as goretex is the only thing that stands a chance of working, as above, overheating then becomes an issue. Showers Pass make close fitting ones, but never tried them. Gore alp-x gt shorts are great, I have them if I need to go out in awful weather, not too flappy and completely waterproof.

If you're worried about your feet then I'd look into overshoes rather than waterproof big tights. No idea if they exist, but it sounds like a one-way ticket to overheating, chafing, general discomfort and nasty infections.