Volvo D12 715/ZF325-1: Filter and Maintenance Questions

I purchased a Cranchi 50 last year with D12s, very happy with how they run for sure. The motors had fresh fluids, but the transmissions weren't done. I've been going through things and have already replaced the air filters and Racors. Late last season the port side motor tripped a "maintenance required" message on diagnostic display. The turbo pressure guage reads .2-.3 bar lower as well. It seemed to build revs much slower than the starboard, but eventually would reach max rpm. My sense is the secondary fuel filters that mount directly on the motor, need to be changed.
1. Can you just unscrew the old ones and put the new ones on, or does the system need to be bled?

2. Late last season when we were going to bring the boat to its winter storage location, the starboard motor didn't want to engage in gear. Did a little research and understand the issue can be caused by a clogged screen in the hydraulic gear actuator. A couple days later, it engaged in gear no problem, but I suspect it might happen again when its more of a big deal. Has anyone done this maintenance on their own, and how difficult was it?

I profess to not be a longtime boater, but longtime like to maintain my own equipment when ever possible. Thanks in advance.

Thanks again for the input. We had quite a bit of work done in the off season, and most of the boat yards here in CT are months out, so I'm both trying to learn more and not get killed on service calls.

I was at McDonnell Marine yesterday, I think it was Dan I might have spoken with. I guess they only have 3 techs and they're all booked out for a couple weeks. I picked up about 600 bucks worth of various transmission, crankcase and fuel filters, which will keep me busy for a bit. Thanks for all the great info.