Welcome to modular synthesizers :-) Did you bought already the rack or not yet? If not yet, I would recommend to go for a bigger case, for example the Intellijel 7U case (2 x 104 HP) or the Doepfer A-100LC9 (3 x 84 HP) as an absolute minimum. So you have some space for extension and you don't have to buy straight away yet another rack (which might come soon enough anyway).
1) Ha, ha, sorry that I have to laugh a little bit here, if you don't really know what to do with it, why you need it then? :-) Perhaps look into this matter a bit more before you decide on it? It's totally okay to start with a half full rack (or even less). Build up experience and then decide if you need a certain module or not.
Or start with one or two of them, if you are happy with it then consider the others you mentioned. Up to you of course but I would take it easy, especially since you notice yourself it might be a bit too much and keep in mind that you planned here a rather small rack so space is (extremely) valuable here to use it as good and as efficient as possible.
2) I feel this is a difficult question to answer. On one hand because I feel one should consider for herself/himself if something is enough or too much on the other hand because... can you ever have enough modulation? :-) The same question goes for oscillators, filters, VCAs, envelopes and LFOs, you usually have too less of them rather than too many of them. Talking about modulation, how about one or two more classical/standard LFOs? I see you planned the Batumi that I guess is certainly not too bad and yes it's a quad LFO, but perhaps a second separate LFO?
Personally, for this rather small setup, I feel the Magneto is perhaps not necessarily? Yes it's a nice to have module but A) it's expensive and B) it consumes a lot of space that you might want to consider to use for other modules.
3) Do you have an external mixer with panning? If yes, you might want to decide for yourself that you don't need panning on your Eurorack mixer. Personally I still would go for a mixer with panning (even if I have externally a mixer with panning too), but as mentioned that's personally, you might think differently about that. That's the nice thing with Eurorack, it's all about flexibility and possibilities but there are no rules that says you must have or you must not have panning. It's at the end totally up to you to decide :-) I feel that's one of the beauties of Eurorack, that kind of freedom to decide for yourself what you are going to do. Which doesn't mean you shouldn't ask here around in the forum for advice though.
4) Ask yourself if a "plain" VCA (that quad VCA for example) is good enough for you or do you really need (straight from the beginning) a polariser? Personally I would start with rather the "normal" VCA first and then later, when build up some experience with Eurorack, decide if you would need a polariser? So if you would want to save some space you might want to let go one of the modules first. On the other hand Blinds has VCA functionality as well, so as long as you don't use the polariser functionality of it, it can be used as a "simple" VCA too. So perhaps start with just the Blinds and then for later consider an additional VCA? Hey this is Eurorack, everything is possible but at the end it's up to you :-)
5) Usually with smaller racks you might not even need a multiple at all. After that (when the small racks becomes bigger or completely filled up) a passive multiple might be for most cases good enough. Only if you know already that straight from the beginning you going to split certain signals into many the same, and that especially for CVs/triggers, then perhaps you could directly start with an active multiple. If you just want to have a multiple for the sake of putting your mind at rest that in case you need one, you got one (that was my approach ;-) ) then go for the passive one. If you then after a while realise you splitting the signal too much then you still can consider an active one. They don't cost the world, money-wise as well as space-wise.
At first sight my above "answers" might not completely been the answers you are looking for. What I however try to accomplish with my replies is that you start thinking about those matters and try to figure out a bit more about those matters and after that you might be able to answer yourself those questions.
You asked what our thoughts about your planned rack is... well to summarise, two things: I feel the biggest issue is that the rack is too small (see already my first comment before point 1). Second... without wanting to remove your enthusiasm here... perhaps you focus a slightly bit less on fancy modules like for example that Magneto and a bit more on some classic/basic stuff?
Since you got already a Grandmother (I mean the device ;-) ) you got there already quite some functionality, so don't worry too much. There is no way that we can plan a Eurorack perfectly without making here and there a "mistake". As long as you are willing to learn from "mistakes" and update/extend your Eurorack accordingly to the gained experience, I think you should be fine and just enjoy the ride :-)
This Grandmother's of yours, I am not hundred percent sure if it's Eurorack compatible, I am afraid it isn't. Perhaps other members can help you there on some details of that. Worst case scenario is that you might need a converter module. So please have that checked.
Good luck, have fun with the planning and for later enjoy modular :-) Kind regards, Garfield Modular.