Duel Wielding Duelist. Crit Chance worth it?

Crit strike gem is certainly worth it since a high level gem adds around 150% critchance. with a base crit of 8% this would mean a total of 12% gain, which for xample can mean getting from 40% to 52%.
How much crit you need from passives depends on your gear and whether you plan on using power charges, but personally i usually aim for around 450% increase from passives.

The goal is to have at least 40% critchance and 50%+ for endgame. How you reach that goal is not so important but my personal take is:
Always use the critchance gem until i can reach that goal without it later on!
Some of my builds use the gem forever, some can switch it for crit damage or something else later on when i get enough crit from passives/gear.

Another important part is getting enough accuracy and base damage as well as attack speed since all of these basically multiply your DpS. spells function a bit differently but for attacks the DpS formula essentially looks like this:
damage x attack speed x chance to hit x [crit]
with the [crit] multiplier being:
(1 - chance to hit x critchance) + chance to hit x critchance x critmulti

As you can see, chance to hit appears twice in the formula since an attack crit is rolled a second time against evasion to check whether it will be reduced to a normal hit, so accuracy is quite important for this.

Example:
Assuming that damage, attack speed and crit multi are the same for both cases we get (numbers rounded to full %):
50% critchance, 95% chance to hit
-> 5% chance to miss, 50% normal hits, 45% crits
65% critchance, 85% chance to hit
-> 15% chance to miss, 38% normal hits, 47% crits

This result is quite interesting since you will notice that both builds have almost the same "effective critchance" (Note: tooltip display only features critchance but not effective critchance!).
In fact, the first build will have a higher damage output since it has almost the same amount of crits connecting through enemy evasion but also more normal hits connecting.

This is also the reason why sword crit builds in general are pretty decent - they make up their lack of raw critchance by superior chance to hit since a lot of sword nodes also feature accuracy.


Regarding Melee Phys support gem:
the gem will only support the initial hit but not the projectiles so it isn't all that good for Molten Strike. Sure, it will increase your tooltip DpS and single target damage by a lot but not so much your clearspeed. Physical Projectile Attack Damage has the exactly opposite effect btw.
I would generally recommend supports that increase both the initial hit and the projectiles like weapon elemental damage or crit instead.

Regarding Chance to Ignite:
No need for it since a crit with fire damage will always ignite ;)