"Unifying the Party"
We can blame Mike Pence and Reince Priebus all day, and we would be right to do so. But even they would carry no sway with Trump if they didn't present a compelling case. The reason Trump has been doing these idiotic things, and much, much more, is that they are pushing the message of unifying the party.
Despite that being an epic press conference, the "unifying the party" thing is quite troubling. What it means, in lamen's terms, is that he is making concessions to "principled conservatives". I don't have to tell you that this is not the platform he ran on. Sure, compromise is part of governing, but not with ONE fringe group of old, dying ideals that was only ever niche to begin with (seriously, no one wants the retirement age raised except people who don't need SS later in life).
He needs to focus not on unifying republicans or democrats, but on unifying the American People. McCain and Graham are dead set on hating him, he'll NEVER win their favor.
How to fix his Marijuana Fail
It's not something he can ignore, and he can't just say Sean Spicer was wrong. It may help, but a lot of people are going to think he simply tested the waters, then doubled back. Middle Aged and old voters will forgive him, but not the younger ones, and the youth vote matters more than most people think because these voters will be around for much longer. Marijuana and the war on drugs are serious issues that these voters will not forgive or forget if you screw up on.
There's only one way to win these voters back, and it's not Medical Marijuana. That ship sailed the moment Sean Spicer opened his dumbass mouth. It would reduce the damage, sure. But to completely undo it, he will have to push full, federal legalization. He's been on the fence about it, but he'll have to now come off and support it, push it through congress if he can, and if he can't, Executive Order.
The EO may be struck down in court, but at that point, his response must be to order Jeff Session to make ZERO Marijuana arrests. That is, make a national stand down order with respect to marijuana. He may not wanna do it, but he got himself into this mess, and this is now the only way out.
How to fix his Transgender Fail
This is a bit more complicated. He can't simply undo what he did because non LGBT voters are more hesitant to support bathroom choice. And to LGBT voters, he'll look, at best like a buffoon who screwed them and only stopped when he faced backlash for it.
The best solution may involve one person bathrooms. These are usually gender neutral. By updating bathroom laws to require one of these at each business, with gendered bathrooms only required in bigger buildings, he can claim to be helping trans people. It won't be enough on its own though, he will have to frame this bill as a pro-trans bill, and will need to explain away his previous move as a way to protect women from the small percentage of people who are rapists that would exploit such a rule.
This way, he can claim that he was just trying to protect women, but also will show he cares that he hurt the trans community and wants to rectify it. This will be widely approved of by non-LGBT voters, but the LGBT community itself will be more mixed, so he will need to make one final move.
He needs to take up Caitlyn Jenner's offer of outreach. And he can't argue with Caitlyn, he needs to listen, and make a deal that he may not necessarily be comfortable with. Perhaps it'll be a new trans rights bill (aside from his bathroom fix), or a federal LGBT relations office. But it can't just be "Okay I invited Caitlyn and we talked", that won't be good enough, some sort of pro-LGBT action will need to follow, or he'll lose the LGBT vote again in 2020.