'98 V70R auto AWD - AWD prognosis?

[RollingThunder]

Hi all, I'm new to the AWD forum so please forgive the newbie questions - I have googled (for hours) but haven't really found any definitive answers! I've owned many Volvos and currently have a '98 V70R so I'm happy pulling the engine etc apart I've pretty much just bought a '98 V70R auto AWD to join the stable, so my thoughts are focussed on the AWD system which is new to me.

My queries/concerns are these -

1. Is the AWD system really so intolerant of mixing new/old tyres on the same axle? I've read on the web that you can't simply replace one tyre on an axle, you have to replace the pair?! Is that really true or just scaremongering? I assume front/rear differences are ok?

2. I understand that the main reason for the AWD failing (apart from potentially differing wheel diameters) is that a (transfer box?) seal fails allowing fluid to escape causing the bearings to overheat? If that is correct, is there an 'upgraded' seal that can be fitted as a preventative measure? How easy is it to replace the bearings? I've noticed that most people disconnect the propshaft and drive it as a FWD car, but I'll want to retain the AWD if it does start to have issues.

3. Is the '98 auto AWD model more or less prone to these issues? The one I have 99% bought only has 110k miles up so is comparatively low mileage (for a Volvo lol).

4. Presumably the rear diff will need regular maintenance too - is any particular oil recommended? I don't mean what is in the owners manual - what have owners found to be the best oil?

5. Finally (!!!) Do we have a resident AWD system guru that I can bug rather than post endless daft questions here?

Many thanks!