I don't think the Zadar is very deep as far as menu diving. I trust DivKid reviews. The Zadar is on my list of "want." Check here.
If the Morgasmatron is too big then try a Joranalogue Filter 8. It's much smaller but you only get one filter (but with 8 different flavors of simultaneously available filters).
Your results may vary when attenuating the outputs of Eurorack into your line level input. You can use a VCA, but you might have some concerns about noise. Best always to check.
There are pros and cons with all sequencer setups. Externals are nice if you want to save HP space and you have a lot of hardware synths that you want to address by DIN MIDI. I have no experience with the varigate. My sequencing options are Expert Sleeper FH2 (to connect to DAW), 1010 Music Toolbox, Befaco Muxlicer, and a Pittsburgh Modular micro sequencer (not a fan of because of complicated button pressing to access features).
Your MI Links offers one 1:3 buffered mult in its package. Most buffered mults I have seen offer two 1:3 mults usually with a normal between the inputs so you can stack 1:6 if you'd like. Going with Links verses a simple 2HP 2x 1:3 buffered mult depends on your needs. No right or wrong... just what works for you.
Attenuverters are pretty simple and most will do the job. I like my Intellijel Quadratts for this kind of utility. But they only work in Intellijel 1U racks. I also have a Befaco Dual Attenuverter. It also comes with offset knobs that give you a lot more control of your signal. I tend to use the Befaco when I really want to explore modulation possibilities... especially if I'm going into an input that doesn't have any pots controlling the modulation input.
Stereo. Worth it? Yes... if you have modules that output in stereo. Mono isn't bad... it's just mono. Having a left and right image gives you a lot more depth. It's very worthy of consideration and should be planned for accordingly.