I suggest purchasing an HO scale NMRA track gauge. This metal tool has almost a dozen reference points inscribed in its surface which will answer just about all of your questions. They cost around $12 and can be purchased on line or at most hobby shops that carry model railroad supplies.

The distance from the rails to the table edge isn’t any fixed amount but a rough rule of thumb is NOT to make it so narrow that if a car or loco were to derail it would end up on the floor. I’d keep it around three inches from the outside of the ties to the table’s edge. Of course, if you have some sort of raised edging along the outside of the table, that measurement could be less if you want.

Hope this helps,

Geof from the US

The side of track to the edge of a cliff or mountain is pretty much your decision. Whatever looks best would be my call although I wouldn’t go any less than six scale feet which works out to be about 3/4 of an inch.