[@BBeast] You are right, I think making the river run with ichor can be subsumed under giving the Seihdh and Seihdhar the properties mentioned in the wiki article. I'll go with that and save 3 FP.
I would say that applying the Monument dynamic to the river and lake would be somewhat forced. While natural things (a tree for instance, or a hill) could easily fall under the Monument model, a river seems too vast to me and not quite destructible by non-divine or extreme magical means. In a similar way I wouldn't call a swamp a Monument if being in it confers blessings, or a cloud if being rained on by it has certain effects on land/people. To my mind, these fall under the more general '1+ Might: Perform some other godly feat.' 
My reasoning for this is partly because to my mind artefacts and monuments ought to be of more immediate use to the god or gods who create them (e.g. a god could use the artefact themselves, or could benefit from the monument's effects). The Seihdh and Seihdhar, however, don't confer any immediate benefits on Seihdhara, but are aesthetically and symbolically nice and have some benefits for mortals, and they will likely have some interesting impact on the cultures and civilisations that grow about them. Maybe I'm overly limiting what Monuments are, but that's my two pence!

Now regarding the river and the lake being two separate bodies as far as Might and expenditure goes, I'd say the lake is part of the river (so the 2 FP that created the river also created its headwaters, they are one in that regard).
The depths of the Seihdh Lake are a Gateway and, as you mentioned, that comes with a set of properties all its own, and I'll expand on that once I have thought on it more.
Now if we break things down a bit, there are effectively two sets of blessings and a 'curse' in place on the Seihdh and Seihdhar together:
[indent]-The blessing granted by the waters nearer the river mouth
-The blessing granted by the waters nearer the headwaters and the upper waters of the Seihdh Lake
-The adverse (possibly lethal) effects suffered by those who drink from the upper waters of the Seihdh Lake without having received training in the Seal first.[/indent]

As each of these is a powerful and permanent blessing/curse on a natural phenomenon rather than a group, I'd think 2 FP for each makes sense, to a total of 6 FP for the properties all together, and the blood would be subsumed under them all generally as mentioned earlier. Of course, these are not curses or blessings in the conventional sense, and so would still fall under the above-mentioned 1+ Might for a godly feat, but the curse/blessing costs give a good guideline for what price makes sense as the effects are more or less curses/blessings only that there is a conduit.

And yup, making things out of bits of gods is very present in ancients myths. I was reading some Mesopotomian creation stories not too long ago and there is [i]a lot[/i] of chopping up gods. What happens there would be the equivalent of all the Mk.III gods conspiring against Archie, chopping him up, then making the different spheres out of his body parts. :| My little river is really tame in comparison. xD