Will seborrheic dermatitis cause thinner hairs by clogging the pores?

This needs an in-person evaluation.  I recommend getting a formal evaluation with a hair loss expert to go over your condition and advise you on best treatment options. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

If you lose hair, and want them back, transplantation can be an option for forehead reduction and correcting  asymmetry on your frontal hair line/forelock.I advice you have a FUE; you would get the desired hairline in just one sesssion of dense packing hair transplant. 

It's certainly possible, although unusual. I'd talk to your doctor about it as well as consider any other medication/lifestyle changes in the past few months that may also be contributing.

Is losing thinner tapered hairs (4.5-5cm) a characteristic of telogen effluvium?No, this so called intermediate hair is somewhat non specific. It does not necessarily mean a person has a telogen effluvium. Androgenetic alopecia needs to be excluded as well as a few other causes. 

Yes, they do. There is really no such entity as a telogen effluvium hair. The proper terminology would be "a telogen hair obtained from a person who is experiencing a telogen effluvium." Hairs that are shed in androgenetic alopecia are telogen hairs. Hairs that are shed in a telogen effluvium a...

It is natural for human to lose daily 5-100 hair shaft in telogen phase, and it is an expected outcome of hair cycle. These hairs appear again and grow back. About 10% of all hair on our heads is in the telogen phase.