this bit about 5 seats is arbitrary.

Fair. I had to put a cut-off somewhere.

Each of those countries has 1-2 dominant parties, with the rest being involved in name only.

In the UK, the Lib Dems have decided which of the 'big' parties sits in government and which in opposition. The Bloc Quebecois is one of the major parties in Quebec. In India, the two biggest parties get 50-60% of the total votes polled, and most governments are composed of multi-party coalitions. Also about a third of states have governments led by a third party.

And as another user already pointed out to you, these countries dont use pure FPTP voting.

And as I pointed out, they were wrong. The UK, Canada and India use pure FPTP, and Russia has three big parties even if you only consider the FPTP seats.

The spoiler effect requires voters to vote strategically, which means no third party viability.

Third parties cannot win only when everyone thinks they can't win. Labour went from a small third party to forming the government in about a generation. The BJP did the same in India. At the state level, there have been many cases of a third party coming from a single-digit percentage of the vote and winning the election.