should I give L!Marth the Distant Counter?

No, he has 21 base resistance and with the res bane he drops to 18, it's not enough to tank magical threats.
Generally, DC + res bane is quite anti-synergetic, plus DC is an expansive skill. If you don't want to keep your Hector gives DC to another unit which has good defences.
I don't know how gamepress can recommend DC on a unit with low res plus a res bane ...

It's a very bad idea, you gain nothing by doing that, it's pretty easy for mages will deal himsignificant dmg, sometime oneshotting, given his bad res rendering his A slot useless.
Also DC won't increase your score in arena in Marth's case since he already has a 300 sp skill in the B slot

You should only give DC to units you really like or has good mixed bulk.
If you like marth to the point that you'll Summoner support him then go for it but if not then don't and keep Hector(he's still good even with that IV since it did'nt touch his atk)

Dis actually great on Marth. If you want to use him in Arena and have the goal of merging him up, that is.

You don't need high res to be a good user of DC, MArth shows that. With his weapon, a +6 res buff goes up to a +12 res buff. And his speed should be high enough to avoid any and all doubles.

There you go.

I would say yes if and only if you buff him with all four tactics. He'll have so many stat points he can function without his A-skill, including 51 spd 44 def and 33 res at neutral without a seal. Could even switch out his default skills (which are quite good) for wrath + noontime for some sustain because he would have enough attack to use galeforce, which means +10 noontime is definitely an option.