By "all these" do mean your following queries?
This limitation comes with exceptions. Fundamentally, if the PIC deems that to fly safely, such as gaining as much alt as necessary to glide to safety when crossing rough country, as we often have to do here in the Snowy Mountains (our airport elev is 3106’), then greater than 5000' but with an absolute limit of 10000'
Once every 30 mins ... NO WAY. A VFR flight must not be undertaken above more than 4 oktas (SCT) otherwise VFR navigation is severely impeded by lack of reference to the ground and regular fixes for dead reckoning. I don't know anyone who can adequately navigate by dead reckoning over unfamiliar territory above BKN cloud. This is not only illegal but a dangerous practice.
Yes, when in flight it can be difficult to determine what sort of cloud cover is over top of you, as looking along the base it all looks pretty thick and close like BKN or OVC. A neat trick is to look at the cloud shadows on the ground. If there are more shadows than sunny spots then you have BKN cover. However, this trick doesn't work so well if you also have lots of high cloud shielding the sun and dulling the shadow effect.
Well you can't stay up there that's for sure . The short answer for this is make sure you remain legal (and safe) and don't get caught above more than SCT cloud. Just make sure you get below the cloud before it closes up, or turn back until you are over less than BKN cover to make your descent. If you can’t fly safely under the cloud then you must change your plans and land at an alternate airfield until safe to proceed to your original destination (If you’ve time to spare, go by air). On the unfortunate occasion that you get caught, due to inattention of your surroundings (situational awareness we call it) DO NOT attempt to descend through just any old hole. First you must be absolutely certain of your position, that is, what is the terrain below the cloud where you intend to descend. Then you have to be absolutely certain of how much clear air you can expect between the cloud and the terrain where you intend to descend. In other words, if you can't be absolutely certain DO NOT attempt to descend through a hole. A further consideration of course is, is there likely to be any other traffic flying about under your cloud ... legally. You wouldn't want to spoil their day with a surprise visit I'm sure.
I hate giving this bit of advice as it may be interpreted as saying it’s OK to do, but in the interest of seeing you get down safely, I should add this. The only type of hole any pilot should consider descending through is something like a long, wide gap. This will give you the option of escaping if as you get closer to the ground you see that the cloud is too low. Power on and climb out through the same gap. Obviously your chosen gap needs to be pretty big, unless you have an F111 power plant on board. Never attempt to drop through a small hole as you will have no chance of getting back out, then you become a sad statistic that the rest of mourns over. As I said before, the short answer is DON’T DO IT.
Rules governing separation from cloud vary for different classes of air space. As a RA-Aus Pilot Certificate holder you are authorised to fly in class E and G areas only. Given that, below 10000’ you must have no less than 5km horizontal visibility and remain clear of cloud by 1500m on the horizontal plane and 1000ft below and/or above it. If at or below 3000ft AMSL or 1000ft AGL (whichever is higher) then the same 5km horizontal visibility but remain clear of cloud and in sight of the ground or water. Obviously, choosing to skim just under the cloud is risky. What if one of those IFR thingies drop through just as you’re passing by? What if your attention wanders to inside the cockpit for a moment, and it only takes a moment, then when you look back out all has gone sickly white?
Trust this helps,
Paul