Tiagra rear derailleur and Deore crankset + derailleur compatibility I want to buy this bike:

http://en.author.eu/adventure/gravel-bike/author-ronin-2018-ua42901301-1154

For most time, I will be fine with the lowest gear being 34x34. However, occasionally I would like to use it for bikepacking. I used to do all my bikepacking trips with my mountain bike with the lowest gears 20 - front x 32 - rear and I always used this lowest gear on long climbs.

Will the shifting work fine, if I put mountain bike crankset and front derailleur instead of the Tiagra shifter and compact crankset? I want to know, whether I can modify the bike for these occasional trips and if having a spare crankset + derailleur is enough.
You problem comes with bottom bracket width and very likely chainline. The Author Ronin has a 68mm BB shell. With the Tiagra crank it uses a 'Road' type Hollowtech external bearing BB. If you fitted a Deore crank, which has a longer axle, you would need a 'Mountain' type Hollowtech BB with spacers. Without checking I don't know if the Deore crank has a wider chainline, but I suspect it does.

I think you are right you would need a MTB front derailleur, as the rings would be further outboard, but I strongly suspect there is a difference in actuation ratios between Shimano road and mountain front derailleurs which means the derailleur would not be compatible with the Tiagra road shifter.

With all the Road/MTB compatibility headaches,  I'd look at options for getting a lower gear ratio with a road crank. The popularity of gravel riding and adventure bike packing on drop bar road style bikes has spawned several 'sub-compact' cranksets. FSA currently makes a 46/30 crank which would get you under 1:1 with the 11-34 cassette.