I can't imagine how a leak from the primary case could affect your gear engagement. The clutch for all intents and purpose is a dry clutch, the oil in the primary is there to lubricate the chain, cush drive and clutch bearing.
A common problem with gear selection is with broken or worn out springs on the gear selector. I had this problem and tried everything, until I replaced the two tiny springs -- whereupon the gears were able to select easily. You can get at the springs by taking off the outer cover, which is a bit fiddly as there are four bolts that have nuts only on the inner side of the forward bulge. Check the selector is not badly worn at the same time.
If your clutch disengages when you kick the engine through, at stand still with the engine off, obviously, your clutch is probably doing its job.
There's quite a good thread on this already here on the site.
That said: First is often a problem for many people when starting. My advice is get the lowest engine revs you can, by using the advance/retard lever, and then roll the bike forwards a shade as you click into first. Works a like a charm. Some people choose to engage second from a standstill and then click into first.