Variable DC power supply

No. You need a fan speed controller - light dimmers are usually asymetrical.

Doesn't matter

Also not linear. I did a test with a dimmer switch direct to the bridge circuit. Got 50 Vdc at the output - no load - in the minimum position. Turned the dimmer up to 1/2 of full rotation, got 60 Vdc. Turned it up a hair more, heard a slight buzzing from the dimmer and then the 1000 uH capacitor blew.

Took awhile to get the capacitor oil off my desk. I guess the 50 volt rating was serious. I'm still interested to try the dimmer, but should have added a step-down transformer.

We ran low voltage lamps via fan speed controls - or look for a symmetrical design to build. If you can find one of the old GE SCR Handbooks it had some good design info. Running a lot of DC through your transformer does nasty things.

no.

not usually.