With IMAP, it will depend on your settings whether the full message is downloaded at the time you ask to view it (ie touching a specific mail message) or has already been downloaded. I guess that is possibly outside your control (will depend which mail client you use).
If the Blackberry is easier and this is for business mail then switch back. I'd be inclined to move completely over to something like Google Mail. You could set it up to 'pull in' your mail from Plus.Net, meanwhile, if you register your own domain (eg a .co.uk or .com) you could send out all future mail messages with a mail address of @ and whenever you spot someone who has used your Plus.Net mail address, send them a polite note asking them to correct their address book and use your new e-mail address.
I think you can give a 'label' (so you could mark new mail from Plus.Net mailbox as PlusNet and then you'd know who used the PN mail address and needed to be told of your new mail address).
If you pulled the mail into GMail, then (a) your Plus.Net mailbox would be kept empty anyway, (b) you'd be able to view mail on any Android phone or via a web browser, and (c) by 'Archiving' a message it would not be shown when viewing the 'Inbox' on GMail, but would still be held (so the mobile would not show it again, unless choosing to see 'All Mail' but the laptop and web browser would still be able to access it).
As for the phone having poor battery life, it surely just needs a mobile charger / mains charger and remembering to put it on charge as often as possible :)
Yes, regarding 'removed from server means unable to download to laptop a week later'. I use several Android phones (Galaxy Ace, Europa, Mini, Huawei and a couple of Sony Xperia Tipo). Some have 1 or 2 GMail accounts, while one (the Galaxy Mini) has about 5 GMail accounts. They are highlighted when they have not been read so I hardly ever delete any (have anything up to 100,000 unread messages across all my mail accounts) but the Android mail will only show the last 2-4 days anyway, so as long as I take a look once or twice a day, I see anything important.
I will usually mark (with a 'star') something I need to deal with later, and sometimes archive (hide from the inbox) something I've read and consider low importance, so it won't be someting I look at again on my phone. If something is important (sent to 'support@mydomainname') it goes to several GMail accounts and my Orange mail box (so one of my mobile phones will bleep as it gets a text alert whenever there is new e-mail sent).
I know some clients hate having a duplicate mail message, but personally, I'd prefer to see it in more than one place (and be able to respond to a client) than to not see it for several hours, which is why I have special treatment for some mail over all the rest, to the extent of sending it to multiple mailboxes, just in case!