Some amps can be bridged, some cannot. It depends on the topography (basically, the type of circuit used) or the configuration (some amps are already bridged internally to get higher power output ... they cannot be bridged a second time).
Are you positive the two fuses are wired in parallel? That sounds strange and doesn't fit any known purpose. Have you confirmed it with a DMM? Are you sure they are not individual fuses each bank of power amplifiers, or are they two fuses wired in series?
The difference will determine whether you need to maintain two 30A fuses or can substitute one 60A fuse. If you can substitute one 60A fuse you will still need to jump the empty fuse socket. In the end I would just forget about the whole fuse thing and carry on as configured. Getting it wrong carries a slight risk of fire or a definite risk of blowing fuses, if the amp would even work at all. I don't see the point.
There is a 3-channel configuration. That would probably be achieved by taking the bridged 2-channel L or R and cutting the bridged configuration, leaving two individual L and two individual R channels. Now they can bridge two of those four into a third channel.
All of the above taken together seems to indicate you cannot bridge the amp to one channel, due to reason 2 (it's already bridged in 2 channel mode).
EDIT: Upon further examination of the speaker output wiring notes silkscreened on the unit it definitely contains four amplifiers (L + L and R +R), with two L and two R, which are bridged in 2 ch mode (L + L and R + R) and although I don't see any silkscreen about 3 channel mode, I'm sure in the manual it would be L + (L+R, bridged) +R configuration. So, no, your mono dreams are over with this amp.
You could simply not use one channel, and configure it in mono mode that way. In fact it's your only option. Most Solid State amps are fine with one channel unused. In that case you would only use the RCA input on the channel you intended to connect to a sub (or whatever) and leave the unused channel RCA unconnected.
Sub frequency signals are usually present equally in both L and R channels of the source, so there is no real penalty in only using one channel as long as the frequencies are truly sub or very low bass ... about 120 Hz or less; above 120 Hz they do become directional.