"A hybrid system with a mix of directly-elected and PR-allocated members calculated on a regional basis ... introduces conditions so complex to describe and administer that the electorate would hate it"
And yet, that is exactly how the elections to the Scottish parliament are run. They use the additional member system and have a "Regional Vote" which covers typically nine constituencies. Most Scottish people seem to be okay with it.
"Under PR you'd have a minimum of 3 parties in every coalition "
Scotland manages it with just two parties, even just one party at times.
"UK-wide PR would annihilate regionally-focused parties in the 3 devolved nations and would be also be totally unworkable for independent parties - independent candidates won 2.0% of the vote so theoretically should have 13 seats"
No it wouldn't, under an additional member system then any regional party or independent that wins a local constituency gets to keep that seat. Whether they are awarded any extra seats depends on how many votes they got in total. In addition, with the Scottish Regional Vote where you are looking at very localised regions then even much smaller parties could get a look in (eg the Yorkshire Party).
In Germany, there was also a rule that if any party won at least three constituencies then they would get the full amount of their percentage share of the vote, even if they got less than 5%. Although they did scrap this rule in 2021.
In reality, under any sort of additional member system you're looking at needing at least 5% of the vote to get awarded any additional seats.
"There would be 16 different parties (based on 2024 results) with at least 1 mp, nine of which have at least 5"
With the Scottish regional vote system, each party in a region needs to get around 5%-6% of the regional vote in order to get one MSP.
In Germany, which runs a similar system to Scotland, there is an explicit 5% lower limit of votes for a party to be eligible to be awarded any additional seats.
"There would be 16 different parties (based on 2024 results) with at least 1 mp..."
You do realise that in the current parliament there are 14 different parties with at least one MP?
"...nine of which have at least 5"
And that there are currently eight parties that have at least five MPs (although one of those parties refuses to sit in parliament)?
Assuming one of the above mechanisms then an additional member system would likely throw up something like this, assuming that the minor parties (eg Plaid Cymru and DUP) still won in constituencies. Under the Scottish system:
Labour - 226 seats
Conservative - 158 seats
Reform - 95 seats
Lib Dems - 81 seats
Green - 45 seats
SNP - 16 seats