Fairly realistic, and, to address your questions:
1. Coal and steel are not a problem. There are mining networks, steel mills, and a good system of trading caravans that carry coal from mountainous regions down to metropolitan cities.
Steel mills also came a little later. Historians place the industrial revolution from the mid 18th to mid 19th centuries, the Bessemer process was introduced in 1857, right at the end of the period. Cast iron (and a specific meaning of wrought iron that was more of an alloy) was emblematic of the industrial revolution.
You have weapon &c technology around the Renaissance level, so good century or so behind the leading edge of the industrial revolution. Mass steel production would seriously impact that.
Of course, a wizard who wanted a fairly large amount of steel could get it, if not by conjuring it, by making a 'steel mill' of sorts work without all the attendant technology. Magically making up for deficiencies in the available processes. For instance, the 'puddling' process of iron could be done with stunning ease and efficiency by a 'magma elemental' or some such strong fire-immune creature.
I'm looking for suggestions on how to deal with things such as monsters, the uncommon spellcasters which do exist, and the growth of technology in a world with things like Demons, Giants, and the like.
Big monsters that are dangerous because they're big I'd expect to be kinda doomed. The kind of firepower that could be brought to bear in the period became substantial. At the end of the period you even had repeating firearms and even Gatling guns. A wizard able to magic his way past problems with earlier designs could bring similar things out even sooner.
Also, and I suppose this really matters to 5e, armies kept getting larger and larger as populations rapidly grew and urbanized. In theory, in 5e a hundred archers or so can kill a dragon - 100 men with muskets is lot easier to field.
OTOH, by the same token, stealthy or shape changing or otherwise hard to track down monsters that used to be given away by their need or compulsion to kill people frequently could thrive in the largest cities, packed with nobody'll-miss-'em victims.
Monsters that reproduce that way could rise to the level of plagues before coming to light.