Personally, I tend to use the melee weapon I'm perked in (you kind of have to in A21) and the ranged weapons I intend to spec into with the exception of I always carry a bow/crossbow, even if I've never perked into them. In my current game I carried an AK for a long time, until I got an SMG (which I had intended to perk into, I just hadn't at the time, though I have now.)
So in my current game I'm carrying a spear/crossbow/SMG and am perked in to all three (but I'm also perked into rifles and explosives, which I use on horde night.)
This is the downside to losing sex t-rex, unperked melee weapons and tools are just far to much of a stamina drain now to be viable even early game. Not to mention it costs double the perk points to get the max effect sex t-rex used to give globally... to only one weapon/tool line. You really feel the lack of sex t-rex on the wrench, while not a weapon, its a tool, and wrenching things drains my stam like mad now.
As for me, I only use the melee weapon I am perked into, as everything else just uses WAY to much stamina for the little damage it does due to the lack of perks. in A20 i'd used different ones earlygame if one was better but in A21 I pretty much stay locked to my initial choice mainly due to the stamina use. As for guns, I don't perk into these into later game, as early game melee is more than sufficent to kill any zombie, even most ferals other than the fatties, you kinda wanna pull out a gun for those. As for guns, I generally perk into the one thats in the same line my melee weapon is as it costs too many perk points to level another stat. I mostly play single player, so I have to do everything myself which basically forces me to always go str first for the mining tool perks, as they are too generally useful to not take.
Also as for guns, I feel the guns need a damage boost as a whole, as they just don't do nearly enough damage I feel. Even perked. Some guns are almost useless like rifles, due to the horrid accuacy of them, I've never seen such a inaccurate rifle in any game, takes way to long for the crosshair to center when ADSing, that its usually better to use any other gun type instead. Pistols are good due to the desert vulture, and the 9mm stuff is a decent backup. Shotguns just destroy very short range but very high damage. Automatics are your typical spray and pray, I just wish they had their own ammo that was cheaper than 7.62mm to craft, as its kinda silly that a automatic is using the same ammo of a sniper rifle, yet doing far less damage.
As for the melee/tool stam use I do have a suggestion, keep a21's system but front load it. First point gives maybe 20% stam use 40% less power attack stam use. Adjust following points so they add up to what it'd normally be at lv 5. I mean i'd gladly spend 1 point in salvage ops to reduced the wrench stamina use by those numbers, but the 8% and 15% power attack for lv 1 now just is not worth the point.
As for the questions, for the first one, I picked I generally only use the weapon I am perked into, with maybe a secondary I am partially into, with exceptions, crossbows and and explosives do enough damage even unperked to be ok to use. Crossbow mostly for sneak attacks and thats about it. For the second question, I picked at early game unperked weapons can do ok other than melee anyway (due to stamina use), but mid to late game you kinda need to perk because they just do too little damage later on without the perks and book series. You start to notice the issue around when you have ferals showing up, a unperked gun just uses up far to much ammo to kill them usually. Crossbow is just too slow unperked reload wise to be useful for anything other than sneak attacks. About only gun that "feels" right across all stages of the game is a shotgun, it could use a effective range boost thou for buckshot, without nerfing its damage.
I'm abstaining from voting.
Difficulty plays such a huge part, but also the amount of cheese you're willing to go for. There's no hard DPS tests or any of the sort, so you can kill everything with a stone axe with a rad remover. You might not have fun doing it, but you could...
In practice, I play on warrior, maybe nudge up to survivalist as the game goes on when the zeds can't keep up at all. 95% in SP, rest in 2 player co-op.
There's two weapons I always carry, regardless of spec; bows and pipe bombs or molotovs. Both do a lot of work without a single skill point.
For guns, it's mostly main weapons only. I often carry two, one with AP, but mostly for avoiding reloads.
I might carry a 9mm to supplement a shotty or a rifle, or to save some ammo from an M60. But that's mostly just to clear out some weaklings, in essence, just faffing about.
I don't carry unspecced assault rifles, just because I kinda dislike them. They do work, but 150 rounds feels too little if you plan on relying on them, and wasting several slots for an alt gun.. nah.
And I Never carry unspecced rifles, as they don't really do much without the skills and even specced they need their own type of prep - so they're never really a functional alternative to the main weapon.
Rifles would be fine if they had proper accuarcy for the gun type, rifles are usually the most accurate guns in a game even for hipfire, when ADSing in 7dtd you have to wait for the crosshair to center before and after a shot, this would be fine if its damage was far higher, but the damage on them is kind of low for all their draw backs, that and automatics have simmlar range to a rifle and are far more accurate than one, which is really akward. If I had a choice i'd mostly just pull out a automatic over a rifle at anytime as its just more accurate, and the fact its auto and a big magazine size means less reloading. Yes I am aware the lever action and sniper rifle exist but they still have the crosshair problems of all other rifles making their bigger magazine sizes mostly pointless.