Q: Transferring between ITunes accounts

All apps/movies are tied to the Apple ID they are purchased with and is not tranferable.

With iOS 5 they have to be downloaded via the Purchased tab in the iTunes store app (or synced from your computer's iTunes), iOS 5 had the Settings > iTunes & App Store > Videos (under 'show all purchasess' heading) setting on it (I think that the setting came in with iOS 7).

Logging into your account on her iPad and redownloading your past purchases may tie her iPad to your account for 90 days : http://support.apple.com/kb/HT4627 (so she wouldn't be able to redownload her own purchases or use iTunes Match or automatic downloads on her own account until the 90 days have completed)

Is that the computer that she normally syncs her iPad to ? If not then she risks replacing the current contents of her iPad with what is in your iTunes library - you would be better off copying the films to her computer and syncing them via that : you can find where they are stored on your computer by right-clicking one of them in the Movies part of your iTunes library and selecting 'show in explorer' on the popup, copying them to, for example, a flash drive and adding them to her computer's iTunes library via the File > Add To Library menu option (you will need to authorise your account on her computer's iTunes via the Store > Authorise This Computer menu option).

Home sharing allows you to listen / watch / copy content from one computer's iTunes library to another computer or iOS device (you can only copy listen or watch on an iOS device, not copy content to it) - the computer and other computer/device will need to be connected to the same wifi network, have home sharing authorised by the same account, and iTunes will need to be open on the computer being shared from.

Setting up home sharing : http://support.apple.com/kb/HT4620

setting up on iOS device : http://support.apple.com/kb/ht4557

using home sharing to copy content between computers : http://support.apple.com/kb/HT4527 (the page refers to music only, but it works for other content types as well)

If you use the same computer then have you already been downloading past purchases without the 90 day association happening - I use one account, but from what other people have posted the association doesn't always seem to happen. Or do you have a copy of them anywhere e.g. external drive that you could add them from ?

in our house we use the same itunes to manage 2 different i tunes accounts, and content - music, books and apps are easily shared.  The only issue come when updateing apps from the pad, that were from the other person's account.  I have only tried sharing a purchased movie, but it also loaded fine on the other person's pad.

A backup doesn't contain any of your iTunes downloads. You could download them on your phone (via the Purchased tab in the iTunes store app), connect your phone to your computer's iTunes and do File > Devices > Transfer Purchases to copy them over to the computer, and then either sync them to your wife's iPad or she can use home sharing to stream them from that computer's iTunes - they will still need to be downloaded for her to be able to watch them on her iPad, but doing it on your phone means that you won't potentially be tieing her iPad or the computer to your account.

You have all 18 downloaded (not just streamed) on your iPhone i.e. they show in its Videos app without the cloud icon on them ?

The cloud icon means that they are only in the cloud, not stored on the actual phone - they need to be downloaded onto your phone (which you can do via the Purchased tab in the iTunes store app) so that you can then copy them over to your computer's iTunes library via File > Devices > Transfer Purchases, and then either sync them to her iPad or stream them via home sharing from the computer to her iPad.

You could, potentially, download them on your computer's iTunes (and then sync them to her iPad or use home sharing to stream them from the computer to her iPad) or download them directly on her iPad (as her iPad is a first gen iPad and only support iOS 5.1.1 she can't stream them from the cloud on it like you can on your phone) - but redownloading them on the computer or her iPad risks tying that to your account for 90 days