Can I modify the fill factor value while a database is in use?
"yes", you can change the fill factor while the database is in use... If you can accept the blocking! I've found that it is seldom a good idea to have it different that 0 (or 100). 

The dbreindex will not work on your heaps, so you should create a clustered index instead. (see for instance this Kimberly Tripps blog post or this https://www.brentozar.com/blitzindex/sp_blitzindex-self-loathing-indexes/ on "heaps with forwarded records"
You may also want to download sp_Bliz from https://www.brentozar.com/blitz/. This will help you investigate what may help your server run faster.