DIY - renewing ceiling insulation and adding sarking

2 years ago I bought an approx 50 year old 3 bedroom house which has loose-fill blown-in fibreglass insulation in the ceiling. I don't know how long the insulation has been there for, but in some sections it has compacted down so much it's not doing it's job effectively (from what I've been reading on the net). The roof is made of ceramic tiles and there is no sarking under the tiles either.

The house has an old ducted 3 phase 3.75HP A/C, which in hot 45 degree days outside in summer, was struggling to maintain a reasonably comfortable temperature inside the house. If I wait until I get home from work in the afternoon before I turn it on, it can be over 35 degrees inside the house, and not get below that until after 10pm. Even if I turn it on before I go to work, it doesn't actually kick in until about 10 or 11am (when the temperature rises to the thermostat setting of say 28 degrees), and the temperature continues to climb up as it can't cool as fast as the house is heating.

There is also no insulation in the horse-hair gyprock (internal) walls which are aluminium clad (originally fibro, which is still behind the cladding).

At the start of last winter I was contemplating installing new R3 (or better) pink batts to replace the old loose-fill insulation, or at the very least, pile up the old insulation into some sections of the ceiling and install the new pink batts in other areas (to save costs on insulation).

I also tried to research sarking on the net, and have discovered there are two types - one with fibreglass backing and one without, with it's main purpose to be waterproofing.

The questions I have are:

1. Is it possible or suitable to install the sarking with the fibreglass backing into an existing tiled roof without having to remove all the tiles to do it - ie: can it be installed effectively on the underside of the rafters (not directly under the tiles or battens as is the usual location)?

2. Would this method (sarking) be of any use to help keep radiated heat out of the roof space (radiated from the tiles)? I have seen the sarking installed in other houses and know the sarking is installed with the silver side facing down, and there needs to be a gap around it. Would that gap be too big if I was to install the sarking underneath the rafters (to avoid lifting all the tiles)?

3. Am I wasting my time and money even bothering with any of this if there is no insulation in the walls?

4. Is my old A/C not up to the task? When I first bought the house, someone told me the A/C is very oversized for the size of the house (it does look like an industrial-sized one). It has two large cooling fans on it (facing upwards) and two large radiators (one on each long side). The radiators get almost too hot to touch on a hot day (however they cool down rapidly after a quick spray of water from the hose, but I wouldn't be able to do that all day every day throughout summer!). I haven't been able to measure if there is any drop in vent/outlet temperature by spraying the radiators with the hose, and how long it changes for (if at all).

I've looked at a few different diagrams on the net of how/where the sarking is to be installed but it still looks confusing...

Now that summer is upon us, I wanted to get this sorted out ASAP (while there's still some cool(er) days left to get into the ceiling space to install it!).

I feel that in the end, the $500-odd spent on pink batts is going to pay for itself in a few years of cheaper electricity bills due to the A/C not having to run as long. I guess I want to know if putting in sarking is going to be similarly cost-effective, or if it's not worth going to the additional expense and time of installing it.