Yes - many hups are built with bathrooms. 

Yes, this is possible. The flue channel should be packed with mineral wool insulation.

To the underside of the ring beam is 110mm, and pad to the joist is 86mm. 

The hup! specification guide has a section on Lateral Forces and Lateral Stability. This outlines the minimum return sizes when an opening is parallel to the host wall structure. If carrying out structural calculations on a specific project, a racking calculation can be done as part of these calculations.

Yes, you can extend your hup! On request hup! can provide extra parts to secure the old walls to the new walls and also the new walls if required too. 

You have 50mm to play with including the insulation and floor finish. Using a threshold this could be increased to 70mm, but the height of any existing internal doors would need to be considered. There are many floor insulation options on the market.
For Tiles you can choose Pro-Warm Tile backer which is available in 6mm (U-value 5.5W/m2K), 10mm(U-value 2.62W/m2K), 12.5mm or 20mm(U-value 1.65W/m2K) and 30mm U-value 1.1W/m2K). 
For other floors Eco-tec floor foam is only 6mm thick, or if you can use a threshold and want to be Building Regulation compliant you could use two Kingspan products. 
Kingspan TF70 which has a U-value of 0.55W/m2K (insulation only) and is 40mm thick +18mm chipboard. Any floor finish would need to be less than 12mm, to come in less than 70mm. 
Kingspan Optim R 20mm which has a u-value of 0.25W/m2K as a floating floor which can be used with a floating floor like laminate or used with a Fermacell 15mm board to adhere other finishes to.