making blind generalizations with regard to energy efficiency about any specific structure is really difficult-to-not applicable.
I've seen older houses built with zero insulation - none - absolutely nothing.
then the 50-80's "standard" stick built 2x4 exterior walls stuffed with fiberglass.
80-90's saw the advent of "house wrap" - Tyvek - wide rolls of spun bond polyester that prevents wind / air infiltration but allows moisture to escape.
newer 'quality' construction of stick built 2x6 exterior walls - more room for insulation
pre-fab exterior panels with foamed in insulation.
windows are a major heat loss area - seals around the edges, double/triple pane, whole host of options available. old wooden windows with no gasketing / sealing would be the #1 heat loss in an old house due strictly to air infiltration.
triple pane insulated windows are the norm in some places - like Scandinavia . . .
air infiltration? when the wind is blowing pretty good:
- hang a sheet of plastic over a window - see if it billows out/in - that's heated air exiting the structure....
- put your hand over an electrical outlet and see if you feel cold air blowing in.... (they sell gasketing kits for outlets.....)
insulation? for 'open' / unused attics - get some additional blown in insulation - double / triple the amount installed by the builder. hot air rises, the ceilings on upper floors are the warmest walls of the house and lose a lot of heat to the attic. btw, if you air condition, it's a double savings - I presume one is familiar with "hot attic" - summer heat from that hot attic is transferred to the room...upper floors are warmer in the a/c not only because heat rises, environmental heat from the roof / attic is assisting greatly in making them warmer.
humidity - measured by a hygrometer, not barometer - does indeed make a difference to creature comfort. low humidity in the summer (perhaps a dehumidifier) means less a/c cost. higher humidity in the winter means less evaporative cooling from the skin, aka lower air temp is comfortable.
"forced hot air" - whether the air is heated by gas, electric, heat pump, oil, coal, whatever - it always the 'most drying' a humidifier installed in the central air handling system is best, room humidifiers less best but work with attention to detail/refilling/etc.
some humidity is good, too much humidity in the winter is not good. the moisture can condense in walls, etc., and driven to excess, you've got a mold problem right quick. if the drawers are sticking or you've got dew/frost/moisture on inside of windows - you're overboard on the humidification thing.
for hot water - prior to last decade energy standards, the major of energy consumed for domestic hot water got wasted in heat loss from the 30-40-50-60-80 gallon tanks themselves. new hot water heaters are better insulated; aftermarket insulating blankets available.
the most "energy efficient" hot water providers are the "instant" types - gas & electric available - they have no tanks, they do not store heated water. they heat the water on demand - meaning you never run out of hot water - downside: they cost - plus install costs for multiple baths/kitchen/anyplace else you want hot water - adds up quick.
the extra equipment/install costs for a 2.5 bath + kitchen + laundry exceeds the breakeven point by about 350% - which explains why the world is still installing central hot water heaters.
>>heating for the elderly:
this is problematic. the elderly are generally less physically active and more comfortable with higher ambient temps. been there, done that. the "best" solution is additional heating in the spaces/rooms where they spend most of their time. 110v oil filled portable radiators worked for us, your mileage / in-laws may vary.