If this is on one ticket - which it should be - then you should be able to through check any bags. They should also be able to check you in, provided you're not too fussy about seat selection, LH is also on Amadeus Altéa. Lounge access at LH would be in their terminal and you would be under Lufty's rules. You could try to persuade them to let you in, but I think I'd pay, on the entertainment scale, to see you try this.