several small signs of something wrong, pics provided

[Rusty Trichome]

C'mon guys...if you are here to help, read her info...

'ello everybody!

Hello. Welcome.

I've had problems keeping the RH up, due to high ventilation requirements (I grow in a very small closet).

Your RH isn't nearly as important as temps. If concerned, an occational mist using ph'd water is fine. I live in the desert, where ambient RH is around 9%, and have found they prefer the lower RH. Especially when flowering.
Do you have adequate ventilation and air circulation?

some fast details:

lights: 250w MH, 24/0, 15 inches from tops
soil composition:65% pre-fertilized soil, 35% perlite
watering schedule: every 3-4 days
fertilizer: BioBizz BioGrow + epsom salts
potsize: 5 litres

Epsom salts every nutrient day? The dingy yellowing is likely a result of salt build-up throwing off your ph, and creating a toxic soil enviornment. (early lockout) A good flush will help release the salts, but wait till the soil drys, or you will risk root rot.
Official Epsom Salts Not Miracle Cure Thread

pic1 & 2
Might these brown spots indicate a calcium deficiency? They are spreading very slowly.

Not likely if it's the only plant showing these signs. What's your water and runoff ph? Are you using tap water?
Looks like you've been adding stuff to correct deficiencies, and have overwatered as a result.

pic3 (top right photo)
These grey/bleached spots I guess are heat stress, but the plants haven't been exposed to very high temperatures. They're not spreading. Guess this is nothing to worry about?

Looks like leaf-irritation from another plant or leaf, to me. (the fans can blow the leaves back-n-forth across another leaf, causing those spots)

pic 4 (the bottom one)
The leaf on the lower right side in this photo shows magnesium deficiency right? I don't get this, as I've used lots of epsom salts to try to cure it (3ml/l).

Or magnesium toxicity.

Also, generally, new growth on both plants have gotten paler recently (brighter green). Could this be linked to magnesium deficiency, or is something else wrong?

Again...back to the lockout. (ph is likely screwed-up)