When I see that same question I see Cleveland going no nO NO NO NO......
1. If I use pressure based absorbers rather then velocity absorbers in tri-corners to treat low frequencies. Does that imply that I will not be able to treat midbass and upper bass in that corner? And what will then the overall effect be compared to using porous materal?
Pressure Absorbers are broadly or tightly tuned to one frequency only. But tuned is tuned. i.e. One centre frequency.
2. What is the best way to combine the two? I understand that it depends on the room and speakers, but are there are general knowledge one can have as a starting point?
Take a look a the design of the Primacoustic MaxTrap, it kinda ticks a couple of boxes. Ditto Corner Traps with Membranes on them, MiniTraps the MegaTraps, again broadly ticking various boxes.
3. I see some are treating the room with only Helmholtz resonators (under 100 Hz) and diffusors (working from around 500 Hz). This should imply that the frequencies between approximately 100 Hz and 500 Hz are not dealt with. Isn't this a problem?
Really? There are some many problems to pick from here. Simple answer, yes.
Back to 1 and 2, maybe you can have your cake and eat it. Broadband porous in Corners, floor to ceiling. Pressure in the middle of the walls tuned to that axial.
4. How vital is it to reduce the reverb time in all bass frequencies?
The BBC had a rule of thumb for an acceptable control room.
No third octave decay time should differ from it's neighbours by more than 10% over a spectrum of probably 100-4K. That would be a nice room IMHO.