Addressing unventilated solum under suspended timber in stone cottage after insulating?

[YodhrinForge]

Explanation of situation follows, skip to the next paragraph for the specific problem if you DGAF. I'm currently running through all the various scenarios of how to efficiently(and cost-effectively) insulate, airtight, and heat my new-to-me late 1800's stone cottage. My initial plan was to rip out all existing suspended timber and break up the existing concrete kitchen extension floor then put in proper insulated slabs with wet UFH, however I'm toiling a bit to figure out how I can handle that all mostly DIY while also living in the property, not to mention the cost which looks to be almost as much per-room as it would be for the materials to do the whole suspended timber floor and slap some phenolic boards on top of the concrete in the kitchen. Not ideal but a significant cost saving. The plan would be to use vapour open membrane to make "cradles" between the joists to fill with wood fibre batts, then an airtightness/vapour control membrane over the top, then a 50mm rigid wood fibre tongue & groove layer to act as a subfloor and level it with the phenolic-topped kitchen floor. I should be able to address the doorway heights largely by fiddling with the trim.

 

The issue this raises however is the solum. Like all these old solid stone wall buildings the space under the floor is intended to be ventilated by the open fireplaces sucking moisture laden air through all the gaps - in this case, between floorboards - in the house and shoving it up the chimney via stack effect and convection. Needless to say when I'm trying to create an insulated airtight mechanically ventilated living space I don't want to deal with cold damp air from the ground underneath, but I'm toiling to figure out a simple solution. I certainly want to avoid knocking holes in the walls near ground level. All I can think of is to run some supply pipes from outside(the back wall of the house has already been opened up in a couple of spots thanks to the late 20thC extension so I should be able to find somewhere to slip a few pipes through) to the front & back of the house, run an extract pipe up the disused chimney, and try to cram a few air bricks into the partition walls below the bottom of the joists so that air can move across. Does that sound practicable, or is there a simpler solution I'm not aware of?

Edited January 22 by YodhrinForge

[Redbeard]

Hi. Suspended GFs do not always appear to obey the rules of building physics! Having tried on a few occasions to use passive stacks to ventilate sub-floors I think I'd be tempted to add a fan somewhere in your plan (and a sump/drain in case of lots of WV  in the 'stack' running back as condensation.