Crab Apple Tree Grafted into an Apple Tree?

Some growers do this as the crab apple branch will act as a pollinator to the apple trees-this is the normal process now days. Vice versa is also possible. However, you may want to have an arborist either show you how or do it for you if you've never done this before.

1. Identifying "tropical houseplant”

My guess would be rubber tree plant but the leaves are similar to magnolia tree which is grown as a houseplant sometimes

2. What apple tree is this?

To my mind 'Yellow Transparent' ('Glass Apple') is a reasonable candidate but I fear there is no way to be sure merely from a single photograph of a single specimen. 'Yellow Transparent' is an offspring of 'Lodi' - so similarities you have noted would be expected. The 'Jonagold' example images on Wikipedia are much redder. But identification problems are many as Malus domestica is the most widely cultivated Malus (Worldwide production of apples in 2014 was 84.6 million tonnes) and has been grown for thousands of years.The genome of apple ('Golden Delicious') has been sequenced - The total number of genes predicted for the apple genome (57,386, including some genes that may be present only in one of the two chromosomes of a pair) is the highest reported among plants so far (about twice the number in humans) so there is much scope for variation. Possibly the earliest tree to be cultivated there are now more than 7,500 known cultivars to choose from so differences can be subtle. In addition they are not necessarily consistent. For example, consider size with regard to just a dozen of the varieties most readily available in shops. "Small" could hint at 'Cox's Orange Pippin' rather than say 'Bramley's Seedling' but while the latter tend to be larger than average ("two or three times the weight of a typical dessert apple") some examples from any given tree will be much smaller than average. Blooms do not all open at the same time and fruit from the central flower, which develops first, is generally larger than from others.There is a further complication in that apples may develop mutations even on a single branch (bud sports). Also, when grown of a rootstock which rootstock can make a difference.And naming is not consistent. The Wikipedia article Apple starts with "The apple tree (Malus pumila, commonly and erroneously called Malus domestica) and essentially the same 'genetic' cultivar may have different names. The National Fruit Collection offers:and includes Yellow Transparent amongst synonyms for Transparente de Croncels:(and 'Snell's White' for 'Glass Apple'). To take differentiation between 'Jonagold' and 'Grimes Golden' (a parent of 'Golden Delicious' and another variety you mentioned) as an example, Orange Pippin considers 66 characteristics:Of these, about 100 (most of the above are subdivided) are shared by the two varieties (eg Fruit size -variable). More than that are not shared ... in general, but individual specimens may share some of these, even if the majority do not. For example for "Flesh (texture)", "Firm changing to Soft" applies to 'Jongold' whereas "Tender, Soft, Melting", "Granular, Coarse, Mealy" and "Moderate" only to 'Grimes Golden'. So not only does the degree of softness appear to be required for some indication, but "Crisp, Breaking into chuncks" is shared by both anyway

3. Has anyone ever heard of a WATER APPLE?

Hey you made me curious so I had to look up water apples. I wonder if it was a different thing in Costa Rica because this one it says that children eat??? Maybe the same name for a different thing. QUIT asking interesting questions!!! I had to stop playing my game and look up water apples. LOL The water apple is the least of the small group of somewhat similar fruits of the genus Syzygium (family Myrtaceae). This species, S. aqueum Alst. (syn. Eugenia aquea Burm. f.), also known as watery rose apple, is distinguished in Malaya as jambu chili, jambu ayer, jambu ayer mawar, or jambu penawar; in Indonesia as djamboo aer, djamboo wer, or djamboo wir. In the Philippines, it is called tambis; in Thailand, it is chom-phu-pa. Description The tree may reach 10 or even 32 ft (3-10 m); has a short, crooked trunk branching close to the ground, and a nonsymmetrical, open crown. The opposite leaves, on very short, thick petioles, are obovate- or elliptic-oblong, cordate at the base and clasping the twig; blunt and notched or short-pointed at the apex; 2 to 10 in (5-25 cm) long, 1 to 6 3/8 in (2.5-16 cm) wide; dull, light-green above, yellowish-green beneath; leathery; not aromatic or only slightly so when crushed. Flowers, faintly fragrant, are home in loose terminal or axillary clusters of 3 to 7, mostly hidden by the foliage. The 4-parted calyx and 4 petals are pale-yellow, yellowish-white or pinkish and there are numerous concolorous stamens to 3/4 in (2 cm) long. Thin-skinned and shining, the fruit varies from white, to light-red or red, is pear-shaped with a narrow neck and broad apex; 5/8 to 3/4 in (1.6-2 cm) long, 1 to 1 1/3 in (2.5-3.4 cm) wide. The apex is concave; bears the thick calyx segments and the protruding, slender, bristle-like style. The flesh is white or pink, mildly fragrant, dry or juicy, crisp or spongy, and usually of sweetish but faint flavor. There may be 3 to 6 small seeds, frequently only 1 or 2, but generally the fruits are seedless. Origin and Distribution The water apple occurs naturally from southern India to eastern Malaysia. It is commonly cultivated in India, southeastern Asia, and Indonesia. In the Philippines, it grows as though wild in the Provinces of Mindanao, Basilan, Dinagat and Samar. It has never been widely distributed but is occasionally grown in Trinidad and Hawaii. It was introduced into Puerto Rico in 1927 but survived only a few years. Varieties In Indonesia, two forms are recognized-one white-fruited and the other red, the color of the latter developing from the base upward. Much variation is seen in the fruits from different trees in Malaya and the flavor of some types is quite acid. Climate The water apple is suited only to low altitudes in the tropics and areas where there is rainfall fairly well spaced throughout the year. Propagation The tree may be air-layered or budded onto rootstocks of Eugenia javanica Lam. or E. densiflora A. DC. Experiments in Hawaii proved that cuttings can be successfully rooted. Culture Little cultural attention has been given the water apple. In Indonesia, when it is set out in orchards, it is spaced at a 20 to 26 ft (6-8 m) distance from tree-to-tree. Season In Malaya there are two crops a year, one in the spring and a second in the fall. In Indonesia, the tree frequently blooms in July and again in September, the fruits ripening in August and November. Food Uses The water apple is mainly consumed by children, the appeal being largely its thirst-relieving character. In Indonesia, the fruits are sold in markets in piles or skewered on slender bamboo sticks. Superior types are sometimes served sliced in salads. According to early writings, a water apple salad is a ceremonial dish for new mothers. Other Uses Wood: The wood is hard and is fashioned into small pieces of handicraft. Medicinal Uses: A decoction of the astringent bark is a local application on thrush.