1. Do I receive flight miles AND 1P credit AND purchased business class bonus?
yes - but the miles will not count toward 1 Million Miles.

[i]2. Do I check in at United or at LH?
LH

[i]3. Do I have access to RCC's or LH lounges or both?
both

[i]4. Is there any benefit to booking the LH flight as the actual LH flight rather than the UA codeshare?
UA/LH: you can't upgrade such a code-share (not with miles nor with vouchers) - you could upgrade a LH/LH with UA miles (you need to ask UA for a paper voucher to do so) or a UA SWU (on standby basis only at check-in or the gate).