You really should be looking at SAP installation documentation to find out its requirements and recommendations, instead of just choosing an OS/cluster platform and blindly assuming SAP will run and be supported on it.
1.)
According to RedHat people at local RedHat Technology Forum last week, they're working on a special "RHEL for SAP" release. Apparently the future versions of SAP will be fully certified only on that specific release version. If SAP says they won't support SuSE in the future, go with RedHat.
As you're starting a new deployment, you should be planning for the future as well. You should be asking SAP people questions like: "How long will this version of RedHat/SuSE be supported? What's the roadmap after that? If I deploy X hosts now, what can I do if more capacity is required in the future? Will it be easy to add more nodes, or must I replace the existing nodes with bigger ones?"
2.)
Again, you should check SAP documentation for requirements and recommendations. The scale of your SAP deployment will be important here too: How many SAP hosts are you going to need? 2? 20? 200?!?
3.)
On Linux, native clusters should not interfere (much) with kernel updates; if a third-party cluster includes kernel modules that aren't available as source code, you may have to delay a security update until the cluster vendor has a corresponding updated module available.
4.)
That depends entirely on the conditions of your "Enterprise license". (Not all enterprise licenses are exactly the same.)
5/6.) SAP support documents probably include cluster configuration examples on their recommended configurations and cluster software. Since the SAP license & support is probably going to be the most expensive part of your SAP installation, you should be
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