When to lube a chain? Should a chain be lubed each time after riding in the rain*? 

What about a 100km mudride (occasional water up to the hubs)? Should I stop in the middle of the ride to re-lube the chain? If so, is it essential to wipe the chain? Note that wiping with a piece of paper will be much less effective than with the cleaning device at home, so some mud and sand will be pushed into the rollers by the oil.

Can dry chain be felt somehow, instead of relying on mileage and weather observations? How to do it?

* - many lubes are marketed as holding a film for months
I think it is a judgement call, but I don't think you can separate lubing from cleaning.

I will basically lube the chain every time after I've cleaned it. Mostly, cleaning will be simply wiping with a paper towel, occasionally it will be the other extreme - I take the chain and the cassette off the bike completely and bathe the lot in degreaser. But after each clean I apply lube.

To quantify, in your situation, I think if I were doing 100km muddy rides, I would clean the chain in between every ride. To lube during a ride.....well I can't see any point in doing that. You'd just be lubing dirt.

In my situation (and I'm a road rider) I will generally clean/lube every week or 10 days, or every 2-3 rides. I count this as standard maintenance, just like pumping the tyres. A "deep" clean, where I take everything off, will be maybe every 3 months or so. I use one of two lubes, depending on season/weather - a light, dry-weather lube (which would certainly be washed off in the rain) or a more sticky wet-weather lube. 

As regards your last point, if you do start to "feel" something, then probably what you'd be feeling is damage that had already been done. I view lubing as preventative, so I'll lube on a time-driven basis rather than an event-driven basis.