Oh gosh, this thread is heartbreaking
I'm so sorry to anyone who has lost a pet because of this.
I'd known that for a while about the Seresto collars and have cautioned many pet owners about them... I don't use flea collars or topical medications for my dogs at all. Some dogs have had a very bad reaction to Frontline also.
I use Simparica Trio for my dogs (chewable tablet that you put in their food). Of course no medication, for humans or animals, is 100% perfect for everyone and there will always be side effects in some of the population, but Simparica has been shown to be much safer than some of the others.
Simparica is not recommended for dogs under six months of age.
My youngest has very bad reactions to vaccines of any kind (she's the only dog I've ever had that was like this and it's because she has some inflammation in her growth plates- not uncommon in very large breeds). She still needs to have her shots up to date of course but she takes an anti-inflammatory/pain medication called Rimadyl (Carprofen) when she has a vaccine. I wouldn't recommend Rimadyl to everyone because in large doses it can cause kidney and liver issues, but if your dog has a pain or inflammatory response to vaccines, it's safe to give it to them when they're having the reaction, but I take my girl off it a few days after her vaccine (it's a prescription med and you have to get it from your vet).
She's getting microchipped at her next appointment and I really hope she doesn't have a reaction to that. There is a lot of conflicting information about microchipping as well but all my dogs are chipped, since I live in a densely populated area and the odds of them getting loose or stolen are not miniscule. But another thing I'd say about that is I would advise against putting your address on your dog's collar tag, since if a thief has that information, it makes it easier for them to change it on the microchip if they claim to be you!
There's also a lot of debate about which collars are the least likely to break off or snap if your dog is trying to get away, and I'd go with martingales- I still use traditional buckle collars too but I use the ones with the metal buckles. There is occasionally room for a dog to slip out of a martingale or a choke but it's less likely. And it's not a good idea to use a choke collar as an everyday collar (I really hate that that's what they're called...) I only use them for training, but I won't use more punishment-based tools like E-collars that shock (when I use E-collars I will ONLY set them to vibrate, never shock). I don't think prong collars are great but if I had to use one I would greatly prefer the rubber one to the metal. The metal ones can seriously injure the dog if the handler does not have the proper expertise, and I mostly see prongs used on very aggressive dogs. I have never used one in my life.
If you are going to show your dog I don't recommend the "designer" show leads because I've seen them snap on larger dogs. I use twisted leather leads and martingale leads.
So that's my (very long-winded as usual) two cents on the safety of flea & tick prevention, vaccines, and collars... hope this was helpful for whoever read this. Now it's time for me to be quiet for once