Is it feasible to use anyweapon with any class?

Feasible is vague, do it, you can definitely finish campaign. This late into a league if you ever get stuck you can essentially brute force your way through content by buying overpowered midrange gear as it's all so cheap now as most people will no longer settle for gear that's not the best of the best.

Sure, but only skills are tied to weapons basically and your starting point on the skill tree is fixed.

Yeah. You can usually get pretty far, but it really depends on what playstyle you want. If you want to just have fun you absolutely can. If you want to make a 1 click screen delete build then you'll be frustrated.

Your weapon depends on the Skills you want to use.

If you want to play a druid that uses crossbows, you could do that.

Your class determines your passive tree and your ascendancy.

You can play anything with any class. Not everything will work equally good, but with enough persistence you can make almost anything playable (to a degree). What your class determines is starting point on the passive skill tree and ascendancy options. The rest is up to you.

For sure you can. You will want to look at the 'tags' of the Skills you want to use and then make sure you are taking the appropriate nodes on the tree to support those 'tags'. Generally if you are using a Quarterstaff you will be looking for things on the tree like 'attack damage', 'melee damage', 'damage with two-hand weapons', 'attack speed', 'area damage', 'area of effect' etc. But you should definitely consider each Skill closely to make sure you are not, for example, trying to scale 'melee damage' for a 'projectile' attack.

OP quarterstaff titans have been done multiple times.

It's possible to make a build with a good clear endgame with any class and any weapon. You just have to adapt your playstyle.

Every character can use all weapons as long as you have the dexterity, strength and intelligence requirements. Of course you can play whatever way you want as long as you are having fun but be aware since you are a druid, the passive tree starting position is determined and you mainly have access to magic nodes and shape shift nodes.

Every class stars at a different point on the passive tree and they have different ascendancies. The reason people stick to their classes weapon (for the most part) is because that side of the tree typically has nodes that support that weapon. However you can do whatever seems reasonable to you. there are lots of builds that use certain mechanics from one class and uses the weapon of another class for example in 0.1 a lot of people played a stat stacking gem legionnaire (crossbow class) but used a specific quarter staff to imitate a monk style build. Anything can work if you find the linking pieces in the passive tree that make it possible

Yes, but you will lose some max damage