The problem is that he was swinging at other Republicans - right from the start as you noted. Whether it is only "wussies" who reacted negatively to that is irrelevant, because a general election vote by a wussy counts the same as anyone else's.Making committed enemies out of people you will later need as allies is stupid, and is part of the reason for the 11th Commandment. So when Trump loses in November, he'll have nobody to blame but himself.
No, this does not compute. The case could easily be made that attacking (R)epublicans buys you votes in the general election, which is what you were specifically talking about.
The calculation that 'he will need them later' is pardon the pun, trumped by the fact that you need to get elected President *before* you get to worry about who you are dealing with. Thanks to the warped electoral map and demographics, any (D)ummycrat can be elected. Almost no (R) can be elected President now. The myth of nominating the perfect dude is over. You must nominate someone who can win ( which ironically is what Bill Buckley actually said and what Rush and Levin completely ignore ).
Beating up (R)epublicans? In the primaries it is a calculated risk. But likely a positive risk. Trump is running a variation of Nixon's apocryphal: "Run to the base in the primary, run to the middle in the general". It is not a bad gamble in my estimation because the most votes are on the other side.
It pains people to accept this fact, but (R) is a minority and (C) is a minority of that. The most valuable votes are direct (D)ummy crossovers, those who voted for (D)umbo last time. Any new or existing vote gets you an (R)+1, but a direct crossover gets you (D)-1 and (R)+1 for net 2. They are twice as valuable as me or you. And this is exactly how you can win the swing states. Trump is showing great strength so far right where he needs to, FL, OH, PA, MI, NC, and WI, is coming up fast. ( Look for USA Today article 3/29 describing movement in PA for Trump ).
Attacking (R)epublicans ( c'mon, it is such a target rich environment! ) gets (D)emocrats to crossover. They will never cross over for a (R) party loyalist. Coupled with his donations to both parties ( constantly decried by Cruzers and other pubbies ) means he gets to run as a legitimate non-partisan, the exact opposite of a bible thumping "conservative" Cruz who they despise.
Even on the merits your comment about 11th Commandment attacking pubbies is wrong IMHO. We have almost NONE who deserve this favorable treatment. I mean why? Because they have an (R) after their name? How much more treachery does one have to take to forget the party?
I suggest everyone call their local election board ASAP and get familiar with the write-in process, just in case the (R)epublicrat snakes steal the nomination. They cannot be rewarded with your votes who so many have died to protect.
Country before Party