Just a quick couple of questions:
a) How can you tell if your hive is getting too crowded?
Bearding is a good indication but 10 frames of drawn comb with bees on all 10 frames means a crowded hive when the foragers get home at night.
Bearding can also signal a switch to swarm mode if it is being done on moderate days and not just the real hot ones.
B) Can crowding take place even if there is plenty of empty foundation in place for expansion in the hive?
One type of crowding can...backfilling the brood chamber with stores making it honey bound. A honey bound hive can swarm too.
C) During the summer months, can you have too much empty foundation is place on a given hive?
Not on a strong hive going into a flow. After the flow yes, as then bees will consolidate the stores. They can draw combs in 3-4 boxes during a heavy flow bult afterward they might reduce that to 1-2 boxes of cured, capped honey, leaving the remainder in draw or partially drawn comb. Also a weak hive can have too many supers for it to protect.
What I'm trying to understand is............ can I prevent swarming, just by ensuring that the bees have plenty of space for expansion?
It is one of the things you can do to help reduce swarming but it must include several other swarm prevention "tricks" as well. There is no absolute cure for swarming. Even splitting hives will not stop swarming and sometimes both the parent hive and the split end up swarming.
Will I create another type of problem by having lots of empty foundation in the hive for the the bees to fill with either brood or honey? (Summer time only)
In the case of a strong hive no, but in the case of a weak hive you can end up with a dwindling hive due to overtaxed resources. Some of the bad things that can happen to an overtaxed hive is robbing, chillbrood, wax moth, and/or SHB.