#1
I have a Peavey 6505 4x12 straight guitar cab. The speakers are junk, so thinking about replacing them with Jensen MOD's, which can be used for both guitar and bass. With this setup, I know I can play guitar on it, but might I be able to hook up a bass head and use it for bass also? Just for home use, no giging.
My gear...

Samick Les Paul (AV6VB) Guitar w/ SD Alnico2/JB
Marshall AVT150H halfstack
Schecter Stiletto Elite-4 Bass
Ampeg BA112 Bass Amp
#2
If the speakers can be used for guitar and bass, I see no reason why there would be a problem.

Since you're only using it at home, you could also consider having 2 sets of speakers in the cab, two for guitar, two for bass... Have separate inputs for them so you can use both at the same time
#3
I already have another half stack with a Marshall AVT150H head and a Legion 4x12 slant cab with Celestion G12T-100's in it, which is what my wife will be using.

I will be switching from guitar to bass pending on what song we are working on, so it would be nice to use the same cab, only switch heads.

Any issues with this cab not having any ports or a tweeter, like most bass cabs have and guitar cabs dont?
My gear...

Samick Les Paul (AV6VB) Guitar w/ SD Alnico2/JB
Marshall AVT150H halfstack
Schecter Stiletto Elite-4 Bass
Ampeg BA112 Bass Amp