phb encounter attack spells
level 1, 2 infernal spells, none with fire or fear keyword
With the 16 to INT, CHA and CON stats given, there are two encounter powers that, while they don't give a bonus based on having the infernal pact, they
do have fire/fear keyworded.
level 13, 2 infernal spells, none with fire or fear keyword
level 17, 1 infernal spell, none with fire or fear keyword
level 23, 1 infernal spell, none with fire or fear keyword
With INT 16, it's easy to multiclass into wizard, which provides access to more fire based powers. There is also the sorceror class (now), which can give more charisma based attacks if the focus is on charisma as the secondary instead of intelligence. Arcane power can help with this to some extent, perhaps.
phb daily attack spells
level 9, 1 infernal spell, none with fire or fear keyword
level 20, 1 infernal spell, none with fire or fear keyword
Do you mean 19? Or are you referring to the power from the paragon path, assuming that no other paragon path would/could be taken? Because the only Infernal spell at 19 has fire, albeit has no attack roll [but does have a damage roll]
If you are referring to the paragon path, the tiefling have a racial paragon that has both the encounter and daily power having fire damage, uses con/int/cha (your choice) as the attack stat.
level 25, 1 infernal spell, none with fire or fear keyword
And a star pact spell, with the con power that has both fire and fear. Both the infernal and star pact spells at that level have no special benefits based on which pact you use. Ignoring powers that don't have the word Infernal doesn't really say anything about what an infernal warlock can or cannot do. There is nothing that forces a character to grab the infernal power at every level, and a character using that feat would likely do things like pick a multiclass and powerswap where appropriate.
also note that infernal warlocks get a grand total of 2 spells with the fear keyword.
Also note that star pact has more, and that star pact has con based powers. Also note that the poster suggested 16 to cha, which opened up all warlock powers.
now, compare that to a race with +2 con (e.g., half-elf), which enjoys a boost to EVERY infernal warlock spell.
However, unless you have a half-elf warlock with 18 to con and charisma, you don't get one that is good at all fire/fear based warlock powers, whether with con and charisma. And while the half-elf would be better at hitting all warlock powers, they would likely have no bonus to intelligence, as they would need to put 13 into str to get heavy armor since they had to put 18's into con and charisma.
finally, remember that a tiefling has to spend a feat for hellfire blood, as opposed to a +2 con race not having to spend a feat for the better benefit.
One feat, in the long run, isn't much of a cost. In the short term, the warlock only runs into a problem at level 9.