I wouldn't make a double change for sure.You have a good point that we will have that little more to play with come May.It all depends on the length of time Cook is out for I suppose.

You can get more replacements but not of the level you need as the team is still a "strength in depth" one... If Doyle is out, you can bring in the Holder's, Kurtz, Bewley, or  Fricke, meaning your team is still strong... Bringing in one of a plethora of 6 pt men still leaves a team full of 6 pt men due to the *strength in depth" idea...  

Perhaps Birmingham fans will confirm but I believe this is the last season the club can run at the stadium anyway?

Kerr will be fine imo. Just needs time to settle in. Don't forgot if he didn't have bike issues in his first race, he would have scored his average.  Also would be good if he got a Prem place, then time on the bike would be similar to other HLs in this league. 

If there was a rider on a 7 point ave available to replace Cook, even temporarily, I'd go ahead & do it. But if not, I'd stick with guests or r/r. I wouldn't make a double change, can't see any top riders willing to come over. Edit: Lynn's total average will drop in May, they'd have 7.30 available to replace Cook.

Gilkes wasn't great for Sheffield last year, so while he was an option, I wouldn't say it's shocking he didn't get picked. Boughen is on a par with Gilkes in the Champ, so Lynn could (rightly imo) think no point in swapping Boughen for Gilkes. Hagon is only a reserve because 1) he has a British reduction, 2) because Berwick have built their team to have a strong reserve. For example Hagon has a higher average than fellow "rising star" Mulford who is in Redcar's top 5. Gilkes may come into contention at some point, but I wouldn't expect any Prem side to change their RS this early in the season.