Is the ear piece filter necessary?

I wouldnt guarantee it but I dont think its vital to the functionality to the phone. I'd say at best it filters any dust and debris that might go through that hole into some sensitive hardware inside the iphone. It might be worth replacing because down the road you might wonder why you cant hear anything when people call.

The problems won't be immediate, but the dust going inside the phone is bad. Lint can hold moisture and bridge connections...causing your phone to not work well, or at all.

Why don't you cut the mesh down and try to slide it into place? It will still look off, but you will have some protection- that is unless you would prefer doing it the right way and disassembling the phone (look at it as an investment).

I found that leaving it off resulted in sounds being really buzzy when talking to someone. It's best to leave it on for dust protection as well as having a good sound.

As above really, it is advised to put it in there purely to protect the insides of the phone from dust/lint. It is easy to fit after you get the screen off just time consuming to get it all apart.

Whoops forgot to install the little earscreen?......myself i wouldn't worry about it, just look at your home pc and how much garbage gets in there. Only prob i can see is if the little fuzzys get in there after about 3 years of buildup and u decide to use your phone in a wet sauna.