Therefore my question, is this legally allowed diverting or landing with less fuel than the fuel figure written in the CFP ?
You can "legally" land with no fuel, however I'm assuming that's not the question! if you "think" you will land with less than final reserve fuel you should declare a " fuel shortage" to ATC and they will more often than not give you priority. If you "know" you will land with less than final reserves you should declare a "mayday".
The destination alternate fuel can be used for any airport, not necessarily the CFP specified airport, of course there could be commercial or operation issues with electing to land at another, no appropriate fire cover, customs, hotac, handling etc. Usually the OFP takes a fuel figure including a full transition or arrival and the historic " mean analysed fuel burn" known to ops is shown on the OFP. Of course the real issue is whether you use your holding fuel before diverting, then run the risk of meeting x number "divertees" or you make an conscious decision to divert and use this to your advantage. Airmanship should prevail.