The sink drain line is OK as is
close the through-hull when sailing to prevent water entering the fridge.

So i had this problem too.  But what i noticed was that if you operate the foot-pump an additional half dozen times after emptying the refer you'll break the siphon.  That fixed it for me.

Does the refrigerator really need to drain into the sink drain line?  There isn't that much moisture coming from there.  Why can't the fridge just drain into the bilge?  My AC unit does that with condensation, and it puts out enough moisture in florida that I have considered turning it into a watermaker!  When I am gone from the boat my dehumidifier also drains into the bilge, then when it gets enough water the bilge pump empties it.  Any reason why the fridge can't drain there too?

The PO took out the foot pump.  Not sure why there seem to be such an emphasis on water in the fridge.  Mine gets almost no water in it.  I think it would be fine without a drain.  Now when I used it as an ice chest before I replaced the compressor system, a drain was very useful.  

I wouldn't treat the original Catalina plumbing and other particulars as 'the word from on high'.  Many of their ideas/experiments turned out to be ineffective (e.g. roll-up door over the 45 degree cabinet on the starboard side from the head door, plywood base under the mast step, etc.).  The fridge pump-out/drain was clearly something that someone in the team had a bee in their bonnet about.  Unless you used ice blocks while on the hook (quite a strong possibility, but then why have a 12 volt fridge unit?), the pump and drain were unnecessary (i suspect this was the reason, a carry-over from the old salts who insisted that they'd always done this back in the day).  

I removed the non-functional foot pump and covered the hole with a white plastic door handle bumper guard from the hardware store.  The T-fitting also came out from the drain line and I ran a longer hose into the bilge.  It onyl gets used when we clean the fridge or spill something in there.  I added another hose from a catch basin under the stuffing box, another from a YETI style drinks cooler for ice under the former navigation table and a drainage hose from the new multi-function A/C/heater/dehumidifier I added beside the drinks cooler (I took out the useless navigator station swivel chair).  To be doubly sure about clearing the bilge, I added a low-profile second bilge pump into the bilge with its own dedicated hose and thru-hull on the transom (works in conjunction with the main bilge pump).  

I added a ball valve to the refrigerator drain line. When in the closed position, we can sail with the sink drain open so the grand kids (non stop eating machines) can still use the galley sink to wash their sticky fingers

I think we purchased the ball valve from West Marine. It had  nipples at each end and I just cut the drain line and inserted the valve with hose clamps. You could probably use one from Home Depot. Technically it should be bronze, not brass, but the sink drain valve at the hull is closed while the boat is at the mooring, so if the ball valve failed, no water would ever enter the boat. Additionally, we use ice blocks in the refrigerator for our short cruises so the valve gets a fresh water flush. Where ever we purchased it, its been 12 years and sill looks like new.

I have a 1994 MK1 and there's a check valve in the drain line from the fridge and it has worked perfectly for me all these years. I have had to replace it twice. Not bad for 32 years. 

We sealed the fridge drain hole with a rubber plug primarily to eliminate a drain of the cold air in the box. We never had a problem with water backing up into the box and our AC tech recommended we seal off that drain hole.