1) It is possible to use any weapon skill/type effectively in hardcoe mode, but to address your concerns:
a: Explosives is easy to use but difficult to specialize in with hardcoe mode. The primary issue isn't blowing your followers away - that's solved by a quick reload. The primary issue is tearing your own limbs apart, because they are harder to heal in hardcoe mode.
b: Recharger weapons are second most efficient per unit of ammunition weight, but the Minigun is an honorable contender as is the Gobi Rifle. I don't use shotguns, so I can't tell you about them. Energy weapons are absolutely not ammunition weight efficient. The most efficient weapons in the game, weight wise, are Love & Hate and Blood-Nap. I highly recommend trying melee if you're concerned about ammunition weight, or you just want a new experience.
2) MRE. ~600 FOD, 3(15) healing, and weighs .3lbs. Practically speaking, I recommend the Gecko Kebab for ~176 FOD, 110 H2O, 2(40) healing, weight of 0.25 lbs at the Survival necessary to craft it. MREs are found in LR, Gecko Kebabs are made from 1 Gecko Meat, 1 Banana Yucca, and 1 Jalapeno for 2 at 60+ Survival. I don't know if Gecko Kebab has the best restoration to weight ratio, but I know its ridiculously common and when combined with Maize, Healing Poultice, and Nuka-Cola one can live and heal on that alone.
3) Bloatfly Sliders & Dog Steaks. Bloatfly Meat (You know where I'm speaking of)+ Prickly Pear Fruit (unlimited supply) and killing those darn Cyberdogs. And of course there's the vegetables of the bio gardens.
There's only four adjustments to make.
1: Take a sip whenever you pass a water source, no matter how irradiated.
2: If you intend to help the PGs take over Goodsprings, don't do it until after you find a second doctor.
3: Carry -300 FOD & -300 H2O of items at all times - in case you become dehydrated after fast traveling or waiting.
4: Switch your companions to ranged weapons, and prioritize getting them armor.
Honestly, you don't need to change anything about combat - the rules for combat haven't changed. If you were relying on Stimpak insta-healing in the first place, you were doing it wrong.