Those IVs are fine on Exeggutor. The difference of 4 stamina isn't much. 2 HP at levels 20-30, 3HP at 31-39 and 4 HP at 40.
4 defence might not make any difference to damage recieved due to the rounding down in the damage formula - 2.99 damage is still the same as 2.00.
Quick moves will probably deal the same damage to your Exeggutor, and charge moves might deal like 1 or 2 damage more compared to a Exeggutor with 15 defence IV.
Zen Headbutt is fine on defence. Obviously it's not as good as Confusion but it's almost as good. It's probably like 90% as effective as Confusion.
That Poliwrath is not worth upgrading for defence. It's Bubble or nothing. Bubble has the strongest DPS of any quick move on defence.
A 0% IV Poliwrath with Bubble will do much more damage than your Poliwrath on defence, because move sets matter more than IVs. Bubble does triple the DPS of Mud Shot on defence, assuming neutral typing, which isn't even close to the damage difference between your Poliwrath's 195 attack compared to the 180 base.
Because Poliwags are so common in your area just catch more until you find a good IV one or hatch one from an egg.
If you find a high level one with lower IVs, just evolve it and don't power it up. A LV27 Poliwrath that has average IVs and Bubble can actually be used in gym combat, unlike a high IV but low level one. It just has lower max potential, which is irrelevant if you aren't going to be powering it up to max anyway.
As for the Slowbro, if it has Confusion + Psychic or Confusion + Ice Beam I would say it's worth powering up for defence. Slowbro is really strong defender, even though it's CP is lower than Exeggutor. It's stats are more suited to defending (higher defence + stamina) and it's moveset can counter super-effective attackers. Confusion and Psychic will deal super effective damage to Poison type attackers such as Venusaur, Victreebel and Vileplume, and Ice Beam will deal extra damage to Grass types, which includes those 3 as well as Exeggutor.
Even though Confusion does less damage to Exeggutor, Exeggutor's quick moves will also deal less damage to Slowbro.
