It will all depend on what stove you intend installing now or in the future. Boiler stoves (most) don't get as hot as none boiler types.
When I do inglenooks I always go for top spec ie: assume that at some point a large hot stove may be fitted there. For this I use Lime putty mix 3 months before the job (you can purchase this premixed, I alway have some on the go as I use it on a daily basis)
You should clean out the pointing then make sure the area is free from dust and wet down well with water. Less is more with lime application, about 8/10mm then allow to dry wetting alittle when needed, more so with hydraulic lime based mixes.
lime based mixes are better IMO because they are a softer mix and are "self repairing" so to speak. Air line crack will knit back due to the curing process.
Problem you face is that you don't have the time to do it for this winter if you intend to use the stove. It may (will) compromise the render as it will not have time to dry out.
Registerplates come in many forms and this is also down to the heat of the stove, I would stick to steel and if you fabricate one yourself make sure you leave an access door to clean the stack and hose if things go wrong. I have one on the outside of the stack for easy access if we have a fire.
