I think that because you have two columns of bolts and they cannot develop horizontal reactions (if they're not bearing against sides of the slotted holes) you should find a solution for equilibrium that relies only on vertical forces (in your case only the two middle bolts would be able to do that). Of course this is not really true as the upper bolts will also develop horizontal reactions due to friction (when these bolts are having a vertical reactions and their bearing into the sides of holes). You could try to assume a friction (slip) factor and try to derive horizontal reaction from the known vertical reaction. Possibly the situation could be such that when under the two middle bolts (which could develop moment resistance only by vertical reaction forces) steel yields beam has already reached its maximum rotation at the connection so that no additional forces develop in other bolts and basically you have all the bolts loaded only in shear.
Anyhow you should also consider a situation where full horizontal reactions could develop and the situation could be the same as for equivalent connection without slotted holes.
These are my assumptions and if I ever take on to design such a connection I think I'll carefully think about them and decide either on one I believe the most or will check all of them.
Best of luck!