Flames would have to make some pre moves or send cap back due to having almost no room

iirc there's no cap after the TDL. They can put some guys on the taxi squad to make it work.

Garland maybe cheap, but giving up a 1st,2nd and 3rd for him and a bottom pairing (at best) dman is way too much.
Hoffman is still out there, and Duclair signed for cheap- both capable of providing the same offense- and the Flames didnt make those moves. Arizona is looking to move pricey contracts- not get rid of cheap ones.

Blake Coleman essentially got a 1st and a 2nd, and he is older and not an RFA. I'd also argue less talented. This is the market for this type of player at the TDL.

Remember, teams add at the TDL... just because guys are still on the market now doesn't mean they won't be adding later.

Your original trade breakdown doesnt mention a TDL for Garland (only Raanta specifically)- but the fact Coleman was signed at a low hit for 1 more year- made him more valuable - he wasnt a rental- Garland could very well be, his future cost is unknown. Also comparing where Tampa was at the TDL this past year vs where Calgary is- isnt even close (although i think Calgary has improved- they arent the powerhouse that Tampa has been for a few years now).
Arizona is a mess of a team, Garland may be moved- but their focus is going to be moving out the bigger contracts (Kessel, Hjalmarsson, Larsson) rather than Garland.

btw- the cap is still in effect after the TDL, it is not in effect during the playoffs. How all this plays out with the 'taxi squad' Im not certain, but if the NHL calls shenanigans on some creative roster management by teams that Bettman doesnt like (Vancouver, most canadian teams outside Toronto)..then expect some heavy 'recapture' penalties for years.