I am curious, what happens if I get a /48 from you and some time later you decide you don't wish to be a LIR anymore. Do I get to keep my /48 somehow (i.e., does RIPE guarantee this)?
Depends on whether the /48 is PI (provider-independent) or PA (provider-aggregable). In the case above (LIR gets /29 and splits it to /48s) it is PA, so the /48 is just a part of the provider's address range, you cannot keep it if the provider ceased to be LIR (or if you wanted to change providers).
But you can instead request /48 PI range in 'sponsoring LIR' mode, which would be independent allocation from RIPE (and subject to small yearly fee), just processed by the LIR. Then you could keep it if you change LIRs.
Your /48 isn't portable, so, no, too bad. If you've got a contract which extends beyond the period where they're an LIR, maybe you have a breach and would seek compensation, but RIPE doesn't care. See also, "What do you mean the guy I leased this apartment from has sold it?".
Most IPv4 space also isn't portable. Even if you have a /24 (enough IPv4 space that it could be portable) it usually isn't, if you actually need portable space you need to make sure that's what you're actually getting, it will probably cost more.
If it's portable then you can just take it with you to another provider.
