there is no such thing as "best". Just whichever works best for you.
details on the mercs here: http://blue.arimyth.com/Mercenaries.html

I would say A2 insight is a good plan. A2 mercs need fhr to be effective as well as dr. I have been putting as much dr/mdr (safety crafts/blood crafts) on my mercs and have been successful thus far (a3 nm) as well as max res and life if possible.
and later you can of course upgrade to infinity and/or dream helm

why do you have to choose between static and nova?

i know you have to max both. thats why i dont get why you have the choooose when you can use both?
until late game and you go to make a tfk or something in a +3 orb but id argue fathom > ftk especially for static because of the range you wont get the benefit from conviction.
the gear used on my a2 merc in norm that let him kill seal bosses with amp was
eth grim reaper (100 % ds) tho im sure you could get better
rockstopper with diamonds
silks with diamond or holy craft tho i imagine crow craw would be rather good with 4 os and ias fhr res
bladebuckle
frostburns
tearhaunch
tho its not ideal it was working rather well for me.
its pretty much the same as building any melee and since you dont neeed dmg first or to meet str reqs its easier to socket gear with diamonds/res and not worry about mana leach or ed as much. blood crafts are nice since they all have life + leach + str all great mods for a2 mercs and gloves/helm have dr expensive to roll them tho

well you dont have to copy exactly cause as i say its not exactly optimal its just defensively gearing a melee. rings ammy can be anything. if you need res slap on a 30 all + life ammy or something similar blood craft rings would be great with str (dmg) life and leach and possible res. you are going to want to get res whereever you can cause it will get harder and harder to get max as the game goes on. lots of uses for not great blood rings too. or safety craft rings.
even frosties can be improved i think what made them so useful was the ctc frozen armor which you can get from crafts

if you were frost nova id suggest a3 cold for the pierce cause you'd be the tank and deal all your dmg in nova range. because you can possibly use static (outside of a3 mercs aura range) and not exactly a tank in your own right (especially early game) i would go with a melee since it doubles as another source of dmg to kill immunes.
a5 mercs are typically more tank and agressive as it reads on blues site but a2 mercs DO atk faster. they have zeal which already gives them more hits and a5 mercs have a slow bash animation which will lead to slower leaching with the same amount of ias. late game if you wanted to switch to an a5 with full IK then you'd have a pretty sweet tank as it gains the full set bonus however early on I would say its more of a preference as they are comparable and I myself have always valued prayer over bash warcry rabies ect. it should be noted a2 recovery mercs have a lot of regen as on top of their prayer pulse which is like 25 at low lvls they get another 80+ life per pulse or something i cant remember the exact number but you can see they have a lot of regen. and insight is just compliments it perfectly and allows for spell spam without chugging blues.

ben- keep in mind that there are lots of leach resistances, so massive LL won't always keep your merc alive in HU like it did bnet. Focus instead on dr, regen, and resists (skull & diamonds) instead.

As for your choice of merc... its all preference. I tend to lean toward regen a2 mercs with insight and lots of holy craft gear for auras. Only reason really is because on HC I want my merc to enhance my char so I'm buff with less chance of dying. With a sorc, I almost indefinitely would do that set up. However, if you're feeling saucy, you can go for a lite a3 merc for the lower res and extra trash clearing. However running alone u will need to be able to kill shit very quickly, as neither of you will be able to tank .

shell as in what weapon? not really - so long as its in one of the ones the rw can be made in. which includes all polearms not sure what else. i have liked putting them in fast weapons since it has little to no ias on the rw itself. ( might be none i cant recall) so a fast base weapon helps. oh and eth. eth being the most important.

1. yes, you need to find eth wpns
2. You can make any eth wpns have up to 5 sockets using the socket items
3. War scythe is a good call for lvl35 as it is fast, which means more hits and more life leech.

definitely you want a fast eth weapon and you can socket it however you need. also I like to have the highest range (5?) on the p.arms, but I'm not sure if this affects a2 mercs much...

ok well u seen my sorc ben bot hwith a2 and a5 merc and to be honest the a5 is better for nova /static for tanking and gear is cheap for him eth steel driver or reg if u can get eth silks for armour then holy crafted rings get some recovery and a holy ammy gets u some med and still doesnt lose out on the mercs ablitly to tank and survive any way hit me up in game can show what mine is using and it cheap to make also prob got an extra thunder staff around if u still need one maybe even some of the other gear u might need