I’ve searched the web for answers, and I think this will work.
If anyone can provide any reassurance, that would be great.

Ive already got dura-ace 10sp road shifters, and I believe these will work with my existing 9sp mtb rear derailleur (deore LX). ???

Will this work with a Tiagra 10sp cassette and 10sp Tiagra chain (or any non Dyna sys 10sp chain??)?

Thanks for any advice.

Yes

The cable pull is the same, so yes it will work.
Shimano chains are not as good as KMC/SRAM though IMHO

I much prefer SPAM when it comes to STIs operating with massive gears at the back. Same pull and you can pretty much go up to whatever pan-sized cogs.
Shimano, with 9sp rear mech, not a chance..

Yes. That will all work fine. 10 speed cassettes share the same spacing and a 9 speed mech will operate with 10 speed STIs. I run Ultegra shifters, XT mech and cassette

As above, on my Fargo i have Ultegra 10 speed shifters, XT cranks and XT 9 speed rear mech, long cage running a 40t biggest cog, gives good spread of gears and has been reliable so far.

Yes it does. If you fancy a shimano clutch mech Wolftooth/Lindarets have some cool widgets (tanpan) to make them work. I’m running Ultegra 10sp shifters, xt/hope cassette and a 11sp xt GS mech nicely.

Thanks for the info everyone. Its great to know I wont be wasting my time setting it all up.
Thanks you.