c-o-o-l deck, backline...
One thing I learned on this forum for taking shots of the heads is to slide an index card up to them. It keeps things nice and bright but reduces the glare and reflection of the flash so you can really see what's going on there, but your pictures are telling.
If you have the funds you can't go wrong sending your heads to JRF Magnetics. They are top notch from
everything I have read and you will, at the very least get a reliable head report, and at the most you'll have them lap and align the heads...you sent them the whole headblock with all the heads mounted to it right?
If you figure that camera out (although your pictures were not bad) and you can get a macro shot or something in close to one of the heads, especially the repro head (the one on the right), I'd be interested to see that. You definitiely have some hours on that deck, and I wanted to see if the head gaps were opening up at all as presented very well on
this page.
I can see its going to need some cleaning up and some repairs, but it is a cool deck.
I bought a "Service Manual" from Ebay which turned out to be a bunch of schematics. Those might prove to be worth their weight in gold some day. But right now I could really use a manual that explains all the functions, cleaning, calibration, degaussing etc. etc.
So are there
just schematics in the Service Manual you got (which, depending on far down the rabbit hole you go with this, they will be
very special...I had no idea what to do with them when I started, but I'm comfortable with them now...don't know what everything does but I can use them to do basic troubleshooting and at least they help me ask a lot of rookie questions

)??
Usually, at least with the Tascam manuals and the Fostex manuals I have the Service Manual includes all the maintenance/adjusting/servicing information along with PCB layouts, parts lists and schematics. Tell us more about what you got.
Sounds like you are, at the very least, needing an Owner's or Operations Manual.
Have to see what other Otari owners have to say about that. Tascam is a great source for their gear, but I don't know about Otari. I'm sure you can find one by Googling.
I'd say step one would include getting the manuals, a demag unit (get the Han-D-Mag...$60-80, but worth every penny and others will agree), some 91% or 99% anhydrous isopropyl alcohol or good quality denatured alcohol and cotton swabs and 100% cotton makeup remover pads for cleaning the tape path, and rolling tape to see what you've got. I see you have some tape...what kind is it??
I hate to be the bearer of a negative opinon, but I think your selection of a mixer is going to be an exercise in frustration and disappointment for what you are wanting to do. It is a fine mixer. I'm sure it sounds nice and it has some nice features, but since it is not an "inline" mixer (no way to independently switch between or route line and mic inputs with them connected simultaneously), and no direct outs (an output on each channel strip that sources directly after each channel fader...without it you have no way to push your levels going to tape...you can use a modified insert cable to use the insert jacks as direct sends but they are pre-fader pre-eq...you'll clip your head amp before you are pushing your level to tape to take advantage of that nice harmonic tape distortion), you'll be limited to recording 4 tracks at a time and micky-mousing the patching for tape returns.
Fine mixer, but $500 of gear that is not really built for what you are doing. You'd be better off with something like
this IMHO. It is what we technically call a recording or inline mixer. You can take the $300-400 you save and get your MRL calibration tape, demag unit and such. The guys at Sweetwater didn't do you any favors or didn't understand what you needed the mixer for.