I'm not mirrajay, but I'll take a stab at this.

Firstly, we had the microchip inserted and the blood taken by a vet in Philadelphia and therefore have the rabies titer paperwork signed and stamped by her. Does this mean that we NEED to have the rest of the paperwork completed by the same vet? (The cats are currently staying nearly 3 hours from Philadelphia with my the in-laws and we would like them to take them to a local vet for the tick/tapeworm treatment if possible)
You can take them to a local vet for the final tick/tapeworm treatment. It does not have to be the same vet that did the rest of the paperwork.
Mirrajay: This is our understanding of the next steps...please could you correct anything you see is incorrect:
Request that the vet (in Philadelphia) sign the EC998 and send it to us (or the in-laws) - Does this show that a vet certifies the cats healthy to travel?
Yes, do that. No, it does not certify the animals healthy to travel (that would be done when you get the tick/tapeworm done). It does show that the vet certifies that the bloodwork/microchip/vaccine have all been done.
Ask the in-laws to take the cats for the tick/tapeworm treatment no more that 48 hours before flying. (Hopefully to a local vet and not to Philly?)
48 hours before check-in for the flight (48hrs before flying would work, too, but not 24 since that's 20 hours before check-in).
The cats are then ready to fly to the UK. They are taken to the airport and board the plane. (We will have pre-booked) Is the Certified EC998 used to board the plane?
I believe the airline checks your paperwork before boarding. The part they'll care about is whatever health certificate they might want, though (APHIS 7001, letter from your vet, whatever). That's what says your pet shouldn't keel over while on board.
It is our understanding that if we fly them with Delta, we need an agent to complete the end of the process when the cats land at London Heathrow Airport. Which paperwork will the UK Customs need to see upon landing? I assume they will need to see the rabies titer corresponding with the correct microchips? (If that's the case, we will make sure the agent has this before the cats land)
If you're flying with them, or picking them up at Heathrow, I don't see why you'd need an agent. I haven't looked into Delta, though, as they're not an option from the city I'm using. The UK folks will want to see all the paperwork - rabies certificate, EC 998 form, etc. You should be able to hand all this in to the airline when you check the pet(s) in so that it travels with them.
Good luck!