First of all sorry for your loss. As a widow and a mum perhaps i can try help with your questions.
Coming to spain will be a big thing for your mum.I went to my daughters in south africa on my own and was terrified -if i,m honest i still get ridiculousely anxious when i go, but not nearly as bad as it was. A lot of it is centred round the airport -the way i counteract this is by being super (over) prepared - making sure i know exactly what the airlines weight allowance, and other rules around baggage is -putting everything into a zipup plastic bag that is liquid or gel beforehand so not trying to do it at the airport lithium camera batterys taped over the top if theyre not in packaging etc and putting all my jewellery onto one chain -long enough so i can just lift it off and put it in the tray without nervous fiddling trying to get rings off swollen clammy fingers etc and getting het up about it.I check in online but print it out (just in case) this year i got brave and got it sent to mobile (but still printed it out just in case)
Also leaving plenty of time to get too the airport -even if that means being way too early -reduces the anxiety build up - and then once baggage is checked in i go too the boards and check gate number, as soon as it appears i then go and and locate the gate -before working back from there to nearest seating/loos/ shop area (because i am ridiculously early)
It sounds crazy if youre used to hopping on and off flights, but if youre not used to that and add too that the extra anxiety that bereavement can create it is much more major -i had been on shorter trips on my own when he was alive,i had been a confident traveller on my own -but crazy as it sounds when he wasnt there at home any more somehow the confidence to do anything at all just went.
On that first trip i was ready to abandon the whole thing in Amsterdam and get a ferry back too the UK -then i got a text from her saying well done for getting so far, and how she was proud of me for doing it - and really looking forward to seeing me -which made such a difference too my attitude -if she was proud of me then i had to continue the journey and justify that pride.
Going to spain there probably wont be a transit airport -but just keeping in touch and expressing how much you are looking forward to seeing her when she is at the departure airport will help her.
While she is there perhaps play it by ear a bit -she might want to spend some time alone so respect that but dont leave her alone too long.I found being gooseberry with my daughter and her husband was quite painful too -which sounds really mean and selfish but we did quite a bit of walking and being the odd one out somehow emphasised i was alone -i,m not saying change your behaviour with your partner i didnt and she wouldnt want that, but try and build in some time with just her -a shopping trip/lunch out whatever she would like.
What to say is a more difficult one. As a parent the last thing you want is to see your children upset -obviousely you are upset because you have lost your dad so she knows that but she might feel allowing you to see her pain will increase yours if that makes sense.
She may not want to express her emotions about her grief -but at the same time not want to feel your dad has been forgotten -so maybe you could throw in something like -do you remember when dad did this --------- sharing things around his life rather than his death might work - i found that much easier - and also by sharing those happier memorys you very slowly start to focus on those rather than the ones around the end of some-ones life. Dont push it if her reaction is negative we are not all the same so i can only speak for myself .and it isnt going to happen in a two week trip this is a long process but something you can build up on even from a distance over time.
Looking forward a bit further. One of the diffficultys i found being mum too adult kids who have left and no longer being carer at home for anyone, was thinking there was no point to my existance anymore - My dog really helped me as he still needed me and i needed something too - but your mum has made a different decision it doesnt mean she will not be feeling she doesnt have a useful role. I wonder with Lucy being old, although she also couldnt bear the thought of becoming reliant on her as her companion and then Lucy dying too. In a way trying to protect herself from more heartache.
Ultimately if she needs to look for work it might not be a bad thing -Even if she doesnt in the end, need to work for financial reasons - part time work might help her with confidence, structuring her day and meeting new people -as well as ensuring that having that bit extra it can pay for unexpected things -that can happen too the house or car without having to constantly worry about them, but also allowing her that freedom -to be able to come back to spain for her holidays or take up a new interest -
Freedom is a very empty vessel if firstly if it means having to lose some-one you love - i know no-one here wouldnt chose to have our loved ones back over our freedom -but it is even emptier if all it brings is more emptiness because you dont have the means to do anything with it.
One last thing -there is nothing pathetic about a text saying i love you -believe me every one will have been appreciated -because sometimes its easy to forget when w,ere eally down that some-one loves us.