Firstly, i cannot register for reduced course since my TPS status is not approved yet(am i correct?)
-- > No , you can.
https://www.federalregister.gov/articles/2015/11/09/2015-28360/employment-authorization-for-nepali-f-1-students-experiencing-severe-economic-hardship-as-a-direct

https://www.gpo.gov/fdsys/pkg/FR-2015-11-09/html/2015-28360.htm

"DHS is taking action to provide relief to the Nepali F-1 students
experiencing severe economic hardship as a direct result of the
earthquake in Nepal in April 2015. These students may request
employment authorization, work an increased number of hours while
school is in session, and reduce their course load while continuing to
maintain their F-1 status."

Is it possible to come back in F1 status again after 6/7 months (for different program/new i-20) from TPS status only?

--> possible yes but decision is soley upon DOS's discrection for reinstaement so check with DSO.

My suggestion ,
Maintain F1 or F2 notmatter what.
With TPS and F2 you can take classes on a cheap community college and then move to F1 and transfer the courses.