I have never "made a declaration" annually or otherwise for any historic vehicle except when registering it as historic in the first instance. I have 8 of them between the cars and bikes and simply tax them free every year.
You do make a declaration, either online or by signing a bit of paper. You do this each year for the MoT exemption as part of the Road Tax process. You are also signing to say the vehicle is of the correct classification, aka Historic status. Maybe be a little more observant next time you do this for one of your 8 vehicles.
Technically you are supposed to notify the DVLA if you make any major mods on a vehicle and they give you guidelines to follow to assess what are "major" mods without great detail.
That is not what the guidance says... have you actually read it? And as stated multiple times in this thread. There is no test or assessment on what is or isn't allowed outside of the guidance, which is a self declaration process. You can contact the DVSA who can give you "advise", but they will not give you a ruling on if your vehicle does or does not need an IVA.
Some mods are allowed and some are very "grey" in their legality.
Unless you are fitting spikes and stupid stuff, no there is no grey area on legality. All mods are allowed, so long as they don't breach Construction & Use or any other Road regulations. What you are badly trying to say is the registration and classification may differ, but this does not make such mod illegal!
Personally I would be a bit nervous about some of the mods mentioned above as being acceptable but the proof would be in the pudding if you were in an accident and both your insurance company and the DVLA were happy with them,
Insurance is completely unrelated. Of course it would be very wise to ensure all modifications are declared. And if accepted and money taken from you, then you are insured.
My own personal opinion would be that a 110 underneath a series shell is in essence a 110 with a bodykit fitted, not a series.
You can't have a 110 with an 88 inch wheel base. That would be something else. Which the guidance clearly states would need an IVA and would not retain its identity. You would be using the 110's chassis number in this situation.
If you fitted a 109 body to a 110 chassis, again this is not illegal, but you would again be using the 110's chassis number and identity.
Likewise, you can put a cutdown body onto a 100" Range Rover chassis perfectly legally too (Land Rover even built a few 100" models). As body swaps are legal and you wouldn't be changing the wheelbase or the RR. You would then use the RR identity.
If you took a 110 and chopped it down to 100", this would be a wheelbase change, which as per the guidance would likely require an IVA if you where to use the 110's chassis number.