I'll be looking primarily at the second option.
How could we alter whales?
There's some links & resources in my answer to Hydrogen Dragons we can perhaps repurpose.
In particular : A cubic foot of hydrogen lifts around 30.8 grams (68 lbs per 1,000 cubic feet)
And I'll be looking at this in relation to the blue whale which is up to 30 m long.
We've dimensions for the blue whale here which suggest we can use a height of maybe 4.5 m for that 30 m length & it's roughly tubular so we can extrapolate that for the width a well, a bit rough & ready perhaps but it'll do for now.
The animal tapers a little at the rear & front so I'll go with 4 m x 4 m x 30 m, that gives us a volume of 480 cubic metres.
If we cut that to 440 cubic metres to allow for actual body mass (this whale dirigibles skin organs & other structures etc. then that's 15538.5 cubic feet of gas with a lifting capacity of 1056.62 pounds leaving you with 1412.59 cubic feet for the non gas volume (the organs & bones) of the animal.
You might find this useful for the conversions from cubic metres to cubic feet.
For reference :
- Density of meat 1.0 g/cm3 or thereabouts.
- Density of bone 1.9 g/cm3 or thereabouts.
- Bone to meat ratio Only one with 'whole' body % is rabbit so use that (28% bone / 72% meat).
Assumption : the given bone / meat ratio is by weight not volume, correct me if that's wrong.
So we've given ourselves 1056.62 pounds of weight that volume of hydrogen can lift & by the ratio above that's 295.85 pounds of bone & 760.76 pounds of flesh & muscle.
Pound to gram conversion
Bone : 134196.68 grams of bone / 1.9 = 70629.83 cm3
Meat : 345077.18 grams of meat / 1 = 345077.18 cm3
Total = 415707.01 cm3, which is 41.57 cubic metres.
Which for a rough concept outline is close enough to the 40 cubic metres we allowed for this.
In summary / So far.
What we have is a 30 m long 4 m wide 4 m deep cuboid (I was lazy should have done it as a tube, but I can't be bothered to go back & redo it so just go with it) consisting of around 1056.62 pounds (479.27 kilo or 0.53 US ton) of skin, flesh, bone & organs, all wrapped around 440 m3 of hydrogen.
That gives us a surface area of 512 m2 that we have to stretch 41.57 m3 of organic animal over.
If we squash our 40 m3 of flesh & bone into a 10x4x1 m cuboid (40 m3) & cut it into 10 m high by 4 m wide (40 m2) slices then 12.8 of them are needed to cover this surface & they'll be 7.81 cm thick.
Dang! suddenly this isn't sounding too plausible, never mind, just hang on & bear with me.
So let's halve that & assume an average skin (which may not be all 'skin') thickness of 3.9 cm, which will leave us with with 20 m3 of flesh & bone to play with for other organs & structures.
OK we can probably do this, but your whale is going to be a lot more fragile than a real whale.
What this means is you're whale can look more or less exactly like a whale & I can imagine it feeding on insect swarms & particulates in the air much as a whale in the ocean feeds on krill.
But the actual meat & bone by volume will be vastly decreased & it's liable to wallow through the air rather than fly & have difficulty flying against strong winds (in fact it probably can't).
Given the gas used for lift a flaming arrow may produce spectacular results if it pierces the skin.